I was doing squats on my first working set 155lb, and after I racked the weight one of the trainers walked over. He told me to be careful, that I go down much further than most (ATG), and thats fine and all but on the way up I apparently tip my chest towards my knees and push up with my hips. At least this is what he told me.
He then grabbed a bar and demonstrated his point, and to me he was partially bent as well. Is the objective to stay perfectly vertical from your waist up. The guy was cool about it, and I'm glad he mentioned something to me before I hurt myself.