The only time I've ever gotten any pushback from passing without warning by another cyclist is when passing a large group of riders at once. There's a route I ride regularly with minimal traffic and generally minimal riders, but once or twice a week I'll come across a group ride of 10-15 riders. I believe all but a handful of then generally have helmet mirrors, so I'll say "On your left!" when passing the last rider in line (with at least 5 feet of clearance) and then just give a finger wave as I pass the rest of them . One of the guys in the back will shout that someone is passing, so honestly I don't know what else they want me to do, but at least one of them always wobbles, makes a surprised exclamation, and shouts at me as I pass. I think they're just shaky group riders in general so any variables scare them. Oh well.