I've always preferred to sit, personally. I think it gives me greater control, as ddw mentioned. We usually practice and brainstorm while sitting, 'cause we're basically pretty lazy, but once in a while, (like right before a gig), we do practice while all standing.
This is relevant to me because in our band, I'm usually the bass guitar player. (Sometimes I play guitar or mandolin, depending on the song). My left shoulder gets achy after a while. I only weigh about 140 lbs, and having a bass that seems like half my body weight hanging off my shoulder is tiring. So I like to practice sitting down.
The funny thing is, standing up to perform seems easy. We always play better to an audience than in rehearsal. More inspired, I guess, but you will catch me massaging my left shoulder between songs.
I've even taking to hanging the bass off of my right shoulder at times, like many mandolin players do, to relieve the stress. I have named this condition: Left Shoulder Syndrome. Though I have a Registered Nurse licence, so far the medical community is not impressed, and doesn't seem eager to add it to the list of diseases covered by insurance. Apparantly it needs to progress to Tendonitis or other recognised MSK disorder before insurance will cover it, and they will then advise you to sit or get off of the bass.
I know this all seems like taking a really long time to arrive at a bad pun, but it is all true. Thanks, BlueJay I've even taken