Why do I keep thinking about Bruce Gilden and his craft/art when reading the last few posts in this thread? Ah yes, probably because he's Mr Famous Street Photographer sticking his camera (and a flash) directly into people's faces to capture the facial expressions of shock, anger, fear, surprise, resentment etc.. Very respectful. It appears to me that the answer to the question of what is appropriate in street photography may differ quite substantial depending on whether you are the photographer or the photographed and also on the level of tolerance of each individual. Don't do onto other what you don't want done onto you could be a guideline - even though there will still be variability from person to person.