Imagine this situation: you are on the street, one person is turned back to you (you don't see the face - could be interesting, could not), and you wait that he/she turn, or half turn - and then maybe it will fit to the whole frame. In my view you must be ready and in split second press the shutter, even risking that photo will be no good. You simply can not have all planned on the street.
For example this photo:
I wanted to have whole face of the man - but simply it was not possible - face was behind the flowers all the time, so I waited, and on the end I got not 100% what I wanted, but anyhow I pressed the shutter.
Or this one:
It would be much better photo is she was looking on my side, but she did not, I let it go , and took the picture anyhow - knowing that result will not be as good as I want it to be.