Really it's situational. If I am on a charity or club ride, I expect people to know the basics of cycling etiquette. Ill call "on your left". If there is a pack riding three abreast and on the center line I'll call "passing on your left". If I'm out amongst civilians I just slow down and call out "coming up behind you" or something that makes the point. I really really hate riding a big charity tour and trying to pass slow riders considerately while some d0uch3 on a time trial bike blows by inches to my left without a sound. Being passed doesn't hurt my feelings. Just let me know.