Although I prefer to use bar chords, I had a tutor video of Martin Taylor at one time, and he shows how he uses his thumb over the top of the neck to help with chords and even bass notes, he also uses the side of his index finger to catch more notes for some awkward chords you could'nt otherwise play. I use this techniqe a lot too. It greatly improves your chord vocabulary.
There's no rule that says what you must and must not do, its up to the player to make up his mind.
They cant shoot you for it