Breathing and Projection are controlled by good posture, which if sat correctly can be as good as when stood upright. Bad slumped posture when standing also impedes projection. People who cannot be heard when sitting down are often inaudiable when standing. Nervous singers will often be put off singing if forced to stand in front of an audience.
Personally in a singaround I prefer to sit where I am, unless I am doing something which needs a visual performance, eg comic songs.
Quack!
Geoff the Duck.