I very rarely will take candid photographs of people who are* homeless. More often, I will talk to the person for a while, and then ask for permission to take their photograph.
On the other hand, I do take pictures of people reacting to (or trying not to react to) homeless folks. Those pictures are often far more revealing, and I never ask for permission in those cases.
*More accurately, who I presume to be homeless. The assumption that one can always tell, by grooming or clothing or other cues, is frequently incorrect. Not to mention rude. At any rate, my working assumption is that people who live in vulnerable social or economic conditions deserve more courtesy — not less.