I hate saying "on your left" personally. I try to say anything else or nothing at all (with plenty of room) but sometimes it comes out anyway.
I don't bother doing it most of the time either.
One half of people ignore you or aren't listening, the other half jump right out in front of you. The one's who give you room to pass are actually aware of their surroundings and don't need to be shouted at.
I couldn't care less if I spook them or not. I do slow down when passing pedestrians, but clearly whatever results in me not plowing into them even at low speeds is the best.