keep your promises

This 19 year old was representing homeless youth at a rally today.

A question for street photographers- any notes on the ettiquite of taking people's photos without asking them first, such as in large crowds, at events, etc? I have always been slightly wary of this, I usually always ask first. People have told me that you "can't get a good photo" or that they will be "unnatural" or too posed. This seems like a fine line to walk between art and disrespect. Ideas or experiances, anyone?