However trite it might seem, when taking candids, I pretty much let the Golden Rule inform my decisions. There are a few limitations I impose on myself, but on the other hand, I do not demand compliance from others.
As has been noted, candids are invariably rude, to some extent at least, but if you have respect for the process and for your goals, I believe there is vindication. Just on an anthropological and historical level, how unfortunate it would be if photos only portrayed the posed or those who first gave consent.
Aesthetically, for me, candid shots usually occur when a human or more and their surrounding environment (the street) conjoin to hopefully produce a compelling photo. The opportunity is often fleeting and unrepeatable.
I seldom seek to capture the emotion or story of the person, because photography is far too presumptuous for such a narrative endeavor unless entering the realms of the documentary or journalism.
As for the homeless specifically, for me, they are off limits. It goes back to the Golden Rule. If I were down and out, maybe I wouldn’t care, or maybe I might even invite it. But maybe not, and maybe not for understandable reasons.
Moreover, while it is not for me to judge the motives of any one particular photographer, I do believe that photographs of the homeless can sometimes recklessly capitalize on the preinstalled drama of privation, resulting in output that is hackneyed if not outright exploitative.
I’m also aware that such photos can help generate needed awareness or simply show a part of humanity as we know it now. Whatever the case, though, disparities suffered in this world should at least prompt consideration; and for me, taking a photo of a businessperson is not the same as taking a photo of a homeless person in regards to these considerations. Again, this is just me.