I usually practice standing, but if I'm working on technique in a certain passage I'll sit because usually I start the metronome at a very very slow tempo and work up. and that usually takes like an hour or longer to get to tempo(depending on the piece that is)
I normally practice just sitting in a very crunched position on my bed. Yeah, that's bad, I know. However, sometimes I may practice at my piano because it has a stand on it.
In middle school during lessons my director used to make me play standing up. Ugh, I hated it.
I practice sitting down in a chair or on the edge of my bed (even though I know I shouldn't). Sometimes at the end of the day I want to play, but I'm too exhausted to stand so I figure this way I get some practice in anyway.