I am trying to build up speed on the electric guitar and notice that as I go faster the neck of the guitar bounces around a little due to the counter-play between my fretting fingers and my thumb supporting the back of the neck. I get more stability if the upper part of the palm of my left hand also counterbalances my fretting fingers. Is this a bad habit?

Also, I notice that many of the fastest shredders have their left thumbs up over the neck of the guitar, which seems to violate some people's rule for building speed. Any thoughts?