I've always done all my practicing sitting down. But when you're going to play live somewhere, you have to rehearse standing up or you'll screw up big time.
Before live gigs, you have to get used to lights flashing on and off, girls looking at you and the other guys moving around on stage... a couple of beers in you and smoke bombs going off, plus you have to come up with some basic moves or all you'll hear after your set is 'well, you weren't doing very much'
Everything has to exagerated X10 or you look boring.