I have two theories on squatting, actually.
After I read SuperSquats, and implemented the idiotic Permabulker mindset/diet that went with it (yes, I drank near a gallon of milk a day, along with a simple egg/milk protein), I noticed that for my 20 rep squats that I had a tendency to bend over a bit too much for my liking, as I was getting a very large ass. I bought some Olympic Lifting shoes (Adidas), and I was able to maintain a more upright posture.
But if I were using a leg press or hack style machine, it was Chucks. Barbell or buffalo bar squats (yes, I have a buffalo bar--it is a very good tool), and I use the Oly shoes.