What kind of shoes do you squat in? How wide is your stance? I can tell you that in a flat shoe, like Converse All Stars, I feel much better doing a powerlifting style squat, but it bites me on any kind of high bar/front squat. Squatting mechanics change too much and I end up on my toes if I'm not seriously warmed up. Even so you can see some heel movement if I'm not really concentrating. In lifting shoes (Oly style) this is not an issue at all.

What kind of warmup progression do you go through before your squats? Any dynamic stretching?