Asked and answered at Digital Photography School.
Asking Permission to Photograph People: “1. I always ask for permission if the person will be the main subject of my photo – I figure that I’m a guest in another country and that I want to behave like I’d expect someone to behave if they were in my home – with respect and friendliness.
2. If a person is a minor subject in a larger photo I don’t seek permission – it’s just not logistically possible to ask everyone on a street if you can photograph them!”