If you need to sit because of a medical problem or you must stand because it improves your singing that is a different story but if you can do both I would suggest sitting when you want the audience to relax and settle down like in a dinning room, a cocktail lounge or a chill-out bar at the beach.

On the other hand I would suggest standing if you want the people to get excited and move or dance.

If you want the crowd to go absolutely nuts try both. Start soft and sitting down and half way through the song kick your stool off the stage and flip out in the style of Jerry Lee Lewis