Ancient History:
One of the things you learn about as a general is conservation of points and making the most of the models you have, and one of the areas where this really shows is in the selection of which members of the command group to include in a unit, if any.
The Standard Bearer grants a +1 to your Combat Result, which stacks with rank bonuses - for a unit of small, expensive troops, the Standard Bearer might be the cheaper alternative to adding another rank; while a Standard Bearer in a unit of missile troops that you don’t plan to see much melee could be considered a waste.
Since only a single standard adds its bonus to a given combat, generals who feel confident about their ability to bring multiple units to bear on a single enemy unit at once might consider only equipping one-in-two or one-in-three of their units with a standard.
Of course, taking a standard risks the possibility of it being captured by the enemy; a very weak unit with a standard is a tempting target for an opponent - but is a very expensive bait.
Musicians are more useful for their bonus to rally a fleeing unit than their ability to decide a drawn fight. For high-Leadership units, this additional bonus can ensure a rally (provided unit strength has not dropped below 25%), whereas with lower-Leadership unit (Greenskins) the bonus simply improves the odds.
Unlike the other command group options, the decision of whether to place a Musician in a unit can depend on the layout of an army. Chaos Dwarf Lords and Chaos Dwarf Heroes have such high Leadership that having them nearby or actually in a unit can make the purchase of a Musician unnecessary for that unit (or nearby units, if the character is the army general). Thus, Musicians make more sense for units that will be away from the general and their aura, or which are not planned to include characters.
Champions share certain attributes with characters - mainly the ability to issue, answer, and refuse challenges; and the ability to be targeted and destroyed. A champion’s immediate benefits are their combat advantage and ability to issue challenges to other champions; unfortunately they are also very vulnerable to armies like Warriors of Chaos, who specialize in issuing challenges and killing champions for their own benefit.
It should be strongly stated that there are only two reasons to take a Champions: when you want the extra attack, and when you want to issue challenges. Taking a Champion and failing to issue challenges with it means you’ve probably wasted your points. Granted, a Champion probably isn’t going to kill a character, even a weak character, but they can challenge and kill or force to retire other Champions; saving your unit from having to face them. If you actually manage to win and overkill, all the better.
Now that the basics are over, let’s look at all of the Chaos Dwarf units and their command groups - by the numbers. When looking at adding a command group, the cost of the individual models must be weighed against the cost of the command group, and the individual strengths and weaknesses of the unit.
Black Orcs
Command Group Cost: Musician 6, Standard Bearer 12, Black Orc Boss 20
Individual Model Cost: 13-14 each
With Leadership 8, Black Orcs enjoy the best Leadership of any Greenskin in the Chaos Dwarf army - and considering how expensive they are, upgrading one of them to a Musician is a solid choice to keep them on the table in case a combat goes awry. Also given how expensive they are, you’ll rarely see Black Orcs in ranks, so with a view of getting the most out of your points upgrading one model to a Standard Bearer is probably a better bet than buying one additional model - after all, if that extra model gets killed by missile fire, it could cost you your rank bonus, whereas another Black Orc would simply pick up the Standard and carry on. A Black Orc Standard Bearer can also take a magic banner, which is nothing to sneeze at.
The Black Orc Boss requires serious thought, because for the cost - 33-34 points - you’re just shy of being able to afford a Hobgoblin Hero. However, a quick comparison of the stats should reveal that a Black Orc Boss is fairly equivalent to a Hobgoblin Hero (sans magic items), and cheaper to boot. While an expensive model, the Black Orc Boss is nearly a minor character, and stacks up very well compared to other Champions, able to decide between 3 S5 attacks (two Choppas, first turn only then S4), 2 S6 attacks (Great weapon, strikes last), or 2 S5 attacks and an improved Armour save (Choppa and shield, first turn only then S4).
Bull Centaurs
Command Group Cost: 10 each for Champion, Musician, Standard Bearer
Individual Model Cost: 20-21 each
As the strongest Chaos Dwarf unit (and most expensive per-model!), and Leadership 9, Bull Centaurs rarely need a Musician. The choice, then, is whether to get a Champion and Standard Bearer or an additional model. A Bull Centaur Champion is not terribly more imposing with its +1 A (though that’s sufficient to kill many lesser Champions in a challenge!), but the Standard Bearer is definitely worth the points, because Bull Centaurs are so expensive that they rarely have more than a single rank-two at most.
In addition, Bull Centaurs are one of the few units in the Chaos Dwarf army that can bear a magic banner - given the choices, that would most likely by the War Banner (25 points for another +1 bonus).
Chaos Dwarf Warriors
Command Group Cost: 10 each for Champion, Musician, Standard Bearer
Individual Model Cost: 9-12 each
Averaged out, with Chaos Dwarf Warriors the choice is between upgrading one model to the Command Group or fielding another model (if you have the extra models to field). Ranks and rank bonuses are important for these models, especially Blunderbusses, so if the decision is between fielding 14 models in (5x2+4) with a Command Group upgrade or 15 models in (5x3), the latter is probably your best best. With Leadership 9, Musicians are fairly unnecessary, and a Champion’s +1 A isn’t quite worth the extra points, unless the unit is equipped with Great Axes; 2 S5 hits can be telling.
Standard Bearers, however, are generally worth the points for their combat resolution bonus (not much use to Blunderbusses, though), and can field a magic banner at additional cost.
Goblins
Command Group Cost: Musician 4, Standard Bearer 8, Goblin Boss 8
Individual Model Cost: 3-5 each
Given their unit size, it is practically a given that a unit of Goblins will max out their rank bonus, making the Standard Bearer less attractive than it would normally be. On the other hand, the Goblin Musician is cheap at 4 points and well worth taking instead of an additional model, if only because those 4 points will help protect the other 60 points (minimum) you’ve spent on the unit. Needless to say, Goblin Bosses are so weak (and cost the equivalent of two additional Goblins) that even with their low cost they aren’t generally worth taking.
Hobgoblins
Command Group Cost: 10 each for Champion, Musician, Standard Bearer
Individual Model Cost: 2-16 each
For most instances, upgrading a member of a Hobgoblin unit to the Command Group is a waste of points. Provided you have the models, the 10 points you spend upgrading an individual model could buy you 5 naked Hobgoblins. Of course, if you don’t have the models, adding a Musician is generally the best option available, because Hobgoblins have weak Leadership and will probably flee (if they don’t overwhelm the enemy by dint of numbers). Hobgoblin Champions are only 10 points shy of a Hobgoblin Hero and have pathetic stats; Standard Bearers work okay in a large Hobgoblin block but are fairly useless for Hobgoblin archers, or if the Hobgoblins end up fleeing instead of fighting.
The exception to the above is for Hobgoblin Wolf Riders or Wolf Boyz; these models average 10-16 points and represent a serious per-model investment, as the choice essentially becomes upgrading a model to the Command Group or adding another mounted unit (provided you have mounted Hobbos you can add). In this case, the Musician and the Standard Bearer both stand out - the former for its ability to help your expensive but low-Leadership Greenskins rally, the latter for combat resolution. Hobgoblin Wolf Boyz are fast but weak, designed to range afield and draw out hidden threats, march block, threaten rears and flanks, and sometimes to mix it up with light cavalry. Hobgoblin Wolfrider Champions are counter to the very idea of mobility, as anyone issuing challenges is obviously fixing to get stuck in, potentially for several turns.
Orc Boyz
Command Group Cost: Musician 5, Standard Bearer 10, Orc Boss 15
Individual Model Cost: 5-11 each
For the cost, there is little reason not to get an Orc Boyz Standard Bearer - for the cost of two models, you get the equivalent of a rank bonus in combat resolution (if the Orc Boyz are already equipped with spears and fighting two ranks deep, this allows increased frontage without losing the rank bonus). Musicians are also cheap, the equivalent of one model for a solid Leadership in the face of rallying.
If you decide to upgrade the Orc Boyz to Big 'Uns, the math changes a little. Musicians become much more attractive, not least because you’re spending more points in the unit and want to protect that investment. The Standard Bearer is still a solid choice for Big 'Uns, as it can take a Magic Banner - preferably the Banner of Slavery.
Orc Boyz Champions are a cut above the rest, but are expensive. If equipped with an additional Choppa, they can be very good for killing the Champions of other infantry units (provided they’re not too heavily armoured) - 3 S5 attacks on the first turn is respectable. A Big 'Uns Champion is far too expensive an investment, though - you’re essentially paying 15 points for +1 A!
Orc Arrer Boyz
Command Group Cost: Musician 5, Standard Bearer 10, Orc Boss 15
Individual Model Cost: 6 each
Command Group upgrades are not terribly useful for Orc Arrer Boyz, since their main point is to pepper the enemy with missile fire and refrain from close combat, where they’ll probably be slaughtered - and if you’re not going for close combat, the Standard Bearer and fairly expensive Orc Boss are nigh-useless. A Musician is still a worthwhile choice, if only because it can rally your Arrer Boyz should they flee due to Fear or Terror.
Sneaky Gits
Command Group Cost: 10 each for Champion, Musician, Standard Bearer
Individual Model Cost: 5 each
With Sneaky Gits, the choice is between upgrading a model to the Command Group or fielding two more models (provided you have the extra models). Sneaky Gits are unique in the Chaos Dwarf army in having Poisoned Attacks, which Wound automatically on a roll of 6. Obviously, the idea with Sneaky Gits is to maximize their Attacks to increase the chance of rolling sixes for automatic Wounds.
While it is highly tempting for many players to upgrade one model to a Champion for that extra Attack, you probably shouldn’t do so unless you have no more models to field. Why? Simple math. Two Sneaky Gits have four Attacks between then, whereas the Sneaky Gits Champion gets only +1 Attack - by taking the champion, you are potentially giving up three Attacks. Of course, this is simplifying the situation, as not all the Sneaky Gits will be able to attack the enemy due to limited frontage, but it is indicative of the problem.
By the same logic, Musicians and Standard Bearers should only become solid options if you run out of models and still have points left - while useful for combat resolution and to help rally, it is far more important to have more Sneaky Gits to make the most of their abilities. With luck, the extra Wounds will cover the combat resolution nicely anyway.
Adding It Up
How you choose your Command Groups is up to you as a general - nobody can tell you how you have to play - but at the same time it helps to review your options and play style to see if you’re really making the best choices. Even though I favor Full Commands in many of my army lists, I realize that very few units actually need them. The Musician in particular can be considered a kind of insurance, either to cover a low probability event (a drawn combat) or to try and save a fleeing unit before it escapes off the table. Champions rarely have the stats to back up their point costs, though the ones that do are to be treasured.
Standard Bearers complement any unit, but the cost of the Standard Bearer (and any magic banner they may carry) should be measured against the unit’s purpose. A unit of naked Hobgoblins intended as bait does not need a Standard Bearer. A unit of Chaos Dwarf Blunderbusses might be better off with that one extra model to keep their rank bonus when firing. A unit of Ork Big 'Uns with Spears and a Banner of Slavery might not be intended to engage the enemy directly, but to cement the front line of raging Greenskins that you’re throwing at the enemy - in which case the cost of the Standard Bearer is the cost of the strategy.
One rule of thumb I have is that Command Groups should never make up more than a quarter of the unit, at most. This is entirely a personal rule; Musicians and Standard Bearers can hand off their jobs should they be Wounded, so there’s nothing to really stop you from upgrading to a full Command Group in a unit of 5 Black Orcs. If two of the Black Orcs die (or your Musician/Standard Beater dies and a Black Orc steps up to take their place) you still have a full command…but that’s all you have. One more casualty and you have to make a difficult choice (two more casualties and your unit can’t rally, but at that point the Black Orc has suddenly realized 'is mates are all dead and he’s been ordered to charge the enemy by 'imself…)
The reason for the rule of thumb is cushion. Consider the player that takes a group of 12 Orc Boyz, and then upgrades three of the models to a full Command Group. You can field a (3x4) formation for maximum rank bonus, the Command Group at the fore, for a small, tight formation. Alternately, you can easily reform to (5x2+2) and take three casualties before losing rank bonus. Exactly nine of those Orcs have to die before the unit cannot possibly rally. Those are nine Boyz to pick up the Standard or kettle drum should it fall. While your command is protecting and enhancing your investment in the unit, the unit is protecting your investment in the command.