Good question, it can depend on the song, the stage, or the audience.
The song; a quiet atmospheric story/song I like to sing sitting down. A chair's best, you're closer to the audience and not dominating with height. Sitting on the front edge of a high stage is great.
If you have a song that starts off quietly and builds, start sitting and stand when it builds. One of my favourite things to do is, when it's my turn to sing and i'm sitting in the audience, I start singing from my seat and walk through the audience to the stage singing all the way. They love it, cos it surprises them. It all adds to the drama and entertainment. Anyway i'm a dreadful show off!
Sal