I understand what you're saying. I don't seek out homeless people to photograph, but if they're plainly visible in common areas, then I feel that avoiding photographing is intentionally misrepresenting the overall scene.(posted 8127 days ago)I have one homeless acquaintance I speak to on a regular basis. Even homeless people generally have relatively private areas they go to when they don't want to be seen.
In the strictest logical sense, though, no, it doesn't make sense to tip them for taking their photo. Give them money if it eases your conscience, but I don't really see it as a reward for services rendered.
Again, I agree with Grant. You're certainly under no obligation to photograph a subject if it makes you genuinely uncomfortable.