Well, it depends from band to band and also it's up down to the individual, I guess.
For example, I believe that Steve Harris is just comfortable wearing shorts and a shirt with no sleeves, and he doesn't do that just because it's the singer gets to be over the top with the costumes.
In Nightwish, Tuomas must be more comfortable with simpler clothes, and I believe Floor choses her own outfits as well, I can't imagine a marketing guy sitting with her to pick up clothes.
One band I find jarring is King Diamond. Ok, he's the guy with all the attention, but still seeing a guy with a full face paint surrounded by people in random clothes, including a guitarist with a baseball hat, kinda kills the whole atmosphere on stage. I don't say they should all use the corpse paint but at least fit a bit more with the imaginery of the band and the stage scenery....?