If 'performing' means doing a song in front of people...
I like to stand because I have more control over muscles and airflow. It matters, because I sing in a place where I have to get pretty loud to be heard. I also usually have too much energy, nervous or otherwise to sit. I've just recently been trying to learn how to play guitar while standing. It's tough because the angles - hands, instrument, eyeballs - are all different.

I'll stay seated if I'm really tired or if I'm somewhere where standing would be awkward. I have sung (choruses) lying down. Late night cabin at the 2001 Getaway - I was dead tired, warm and comfy and wasn't gonna move for nuthin'!