Standard Bearer - Only take one of these in units that you expect to take into ONLY winning combats. Putting a banner on a unit means that the unit will either be deciding its engagements (cavalry for example) or it is a unit that will not be able to decide its engagements but needs to (or has a chance to) survive for success. If a unit can be taken out by the enemy's fast moving support units, then putting a banner on the unit is asking for trouble. Likewise if it is a unit like a 10-man shooting group that you'd prefer the enemy ignore, putting a banner there begs for more attention.
Musician - Only put this on a unit that you might be taking into close combat, a unit that you might flee or tactically retreat with, or a unit that has minimal leadership that might fail a terror test initially and need to rally to come back. In other words, most units that can take a muso, should take a muso.
If you're talking about your big infantry blocks going head to head with other big blocks, you might be looking at some draws and you better not get caught without a muso. Should your block survive a big charge from a monster or chariot and it comes out as a tie, then with your muso you know that you get to win that combat and possibly make them flee or crumble a bit. In the case of fear causing enemies this could make a huge difference, should they have a muso while you left yours at home.
Talking about cavalry, yes we have a very high leadership, but remember a movement of 7 will have you rolling three dice and dashing for the nearest edge, which is something you want to be certain does not happen. Of less concern is combat itself for cavalry, as if things go well you should be only declaring favorable charges (winning by a fair margin), but you never know what the tides of battle will throw your way.
Detachments and harassment units should get them as well, since these are units you are almost certain will be fleeing for tactical advantage. Now some will contend that throwaway units shouldn't get even musicians, because they just become that much more expensive... But if a unit can be "thrown away" twice in a game instead of just once, isn't it worth paying a few more points to help them rally and keep doing their job?
A final thing worth noting is our cold-blooded nature... an extra point toward rallying adds a lot more certainty of rallying than it does when warm bloods have a musician.
Champions - Hero protectors in large units, possible liabilities in others. I like big blocks to all have champions, as they tend to function better when they remain "big". When a big nasty charges and does special damage on the charge, it is nice to have the champion stand up bravely and get slaughtered. This helps preserve your numbers, though you are just as likely to lose combat (unless the enemy manages more than 6 wounds, in which case challenges save you those extra CR as well). For cavalry you're just buying an extra attack really, and if you plan on putting a character in the unit the champ doesn't even count towards "Look out sir!" In skirmish and shooting units they tend to be a total waste of points, so take a musician instead if possible.