I signal when I think it will make me safer, or if it helps a driver. If I'm turning right at speed, I'd often rather keep my hands on the bars. Sometimes if I'm about to take the lane, I'll stick my left hand straight out, palm facing back, in a combo "moving left/don't pass" gesture. I also use my right arm when I signal right turns. The left arm thing is because you can't use your right arm to signal when you drive a car.