It depends on how much room you have to fill and whether there is a sound system or not. Also if I am playing guitar, I always sit, cos I don't have a strap either.
If you need to project and fill a large room, standing is the only way.
The sound system part varies with the performer, anyone can fill a room with a sound system, it takes lungs to walk past the mike and still fill the room. It can be effective if you can do it though, Tom Lewis does it frequently, though not for every song.
JohnB.