I don't introduce myself, since that undercuts the candid element, but moreover, when I take a photo of a person, it's usually because the person is situated in a visually appealing way, and often in a way that lasts for only a few seconds. That is, the photo is often of a fleeting situation as much as it is a photo of a person in particular.
As for negative reactions, I've only encountered about two or three in the past eight or so years. It's a matter of learning to be quick and discreet. A disarming smile if noticed also helps.
Plus, if someone appears as though they would be particularly agitated, then I avoid the situation. Other photographers set their own limit. I have a number of self-imposed restrictions as well as just plain old inhibition...for every one photo of a person I take, I probably skip another five to eight just out of nerves.