Anonymous 'faces in the crowd' thing, where no one person is the focus of the picture, I'd go ahead and snap away. But if one face is the point of the picture, then ask - either before or after, if you don't want to make your subject self-conscious.
It's not photography, but I work in radio and frequently have to go up to total strangers and ask them questions and so on. It's nerve-wracking at first, but I've gotten used to it. Be polite, say 'excuse me', and most people will be civil in return, is what I've found. They may say no, but they'll do it politely; they may say yes, and be rather flattered.
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