Well I do know a double bass player who sometimes plays in the laid back (literally) position.
Cellists usually sit down, though one intrepid friend managed to play at a brisk march when we did our Sport Relief mile down Edinburgh's Royal Mile: she strapped her cello on with a guitar strap! ( She did say the intonation wasn't that great!)

I also prefer standing for singing because of the breathing thing.
Easier to play piano/keyboard sitting, and bodhran is much better sitting: it may look more dramatic standing up holding it in the air and just whacking it, but you really can't do the left-hand stuff properly while standing.
Can play guitar or melodeon either way, tho' better with 2 straps rather than one for standing with the latter.
And in a band, I don't think you all have to do the same:just whatever is most comfortable/ suits you or your instrument.