I signal if I can, but negotiating a turn with one hand on certain roads can be tricky. (Like our street car track riddled streets here in Toronto.) In which case I signal about 10~20m ahead of time for as long as I can, then put both hands on the steerer, check my blind spot to see any cars trying to cut me off, and then proceed into the turn.

Oh, and I don't just stick my arm out, I actually point my fingers like a hand gun to show my intentions.