It depends. If I'm in a trafficky area and I'm passing someone at close quarters, I'll call out. If I can give the other rider wide berth, say 10 feet or so, I don't. I feel a certain degree of situational awareness is every rider's responsibility and as such, it's on each cyclist to look behind and to the left before making a maneuver that might cut another rider off. I'm much more likely to call out when passing pedestrians, because I assume they're paying less attention to their surroundings given the more relaxed nature of walking. Plus, frankly, "on your left" always struck me as just an attention seeking ploy from a certain type of rider - like they want to make sure you're fully aware of how fast they can go, how fancy their bikes and biking outfits are and so on. Instead of "on your left", maybe try adjusting your speed to something more appropriate to the location and traffic density instead...