Sometime the only rules that apply are propriety.

So, I've made all kinds of pronouncements about how one should do street photography but here's one situation that falls outside my strictures.  I saw the face and wanted to do a quick portrait but she was in the wrong light.  I walked up and asked, in very broken Italian, if I could take her portrait and if she would mind moving about twenty feet to the other side of the street so I could take advantage of the overhanging structure to shield her from the direct sun but close enough to a bright wall so I would have some direction light on her face.  It was near dusk and she was also illuminated by the light fixture in the overhanging structure.  The whole process took about three minutes.  She was amenable but guarded and that was exactly the look I wanted.

Rules are helpful in defining the boundaries that you must inevitably step over to do art.