I have found that I do better if I rehearse the same as I perform. In other words, if I will be performing standing up (I generally do), I rehearse standing up as well. That way I'm in familiar territory.

I find I can sing better standing (I understand the "pushing the voice by sitting" thing, but don't do that myself, so I can't vouch for it). As for the playing part of the equation, if I always stand to play, my hands will always be suspended over the strings at a consistent angle. In the past, I used to practice sitting down, then stand for the gig, and wonder why my instrument felt so different. Now I stand all the time, and don't have that problem.