Generally, I prefer standing up to play and sing, because I find it easier to keep the beat when I put my whole body into the rhythm. I also sing better standing up. But if my back or feet are bothering me, I sit.

I also sit when standing seems to separate me from the group (in a sing-along or entertaining a very small group).

If I have only one mic and no direct-in for my guitar, I sit, so the mic can pick up both voice and guitar.

I do sing lying down sometimes, Jerry -- but usually not in public. ¤;-)