I always ride on the open road; there aren't any bike paths around here. If I'm approaching somebody who looks like they know what they're doing (i.e., no problems maintaining a straight line), I'll usually say something, whether it be "good morning" or "on your left" just to let them know I'm there.
If it's somebody riding "sketchy", I'll blow right past with a wide berth without saying anything. As has been said above, saying "on your left" usually produces a reaction of them looking back to see me, and most people can't look over their left shoulder without steering left as well. So by not saying anything, I'm getting them to do what I want them to do -- just keep doing what you're doing.