My natural inclination is to stand. I guess it's from my working background: standing says "Look at me: I'm in charge here!" (It's similar to the one with the flip chart marker pen becomes the focus of the meeting and can dictate the agenda.)

However it can really change the whole vibe of a gig to sit down. Much more relaxed and intimate. As for singarounds etc I tend to sit, unless it's a real rocker!