At this point in time, I'd say unless you have something utterly new to communicate, don't. It seems like virtually everyone at some point or another in their photographic journey (or at least one person in every Photo 101 class) has done a photo essay on the homeless, and virtually all of them are exploitative or at the very least fail to communicate anything meaningful about the people or their situation. If you want to make a difference through your camera, then spend some time volunteering at a shelter or a soup kitchen, get to know the people, then photograph them as whole people. Otherwise, you're not contributing anything - you're just creating jerk-off "artporn" of the worst kind. It's certainly not illegal to take whatever photos of the homeless you want - they're on the street, in public, with no expectation of legal privacy. But as they don't have any place to go to exercise moral privacy, taking advantage of their lack of legal privacy is to me extremely low.