When I started weight lifting in high school, I was always told to go ass to floor. Thats the way I trained up until 4 months ago. I was squating one day, I had warmed up properly, stretched, etc. As I came up i felt a tear in my left knee. The pain was tolerable so I kept training as I was. A few weeks later I tore my right knee. Needless to say it got worse as time went on. I finally took a few weeks off, and came back lifting light on quads. Now I keep my range of motion a little above parallel. Unfortunately I didn't learn about range of motion restrictions on any exercise until after this event happened. Trust me when I say this: limiting range of motion on squats to parallel or a little above parallel is more for tendons and ligaments rather than muscle obviously.
So, those of you that lift heavy (6-8 reps) on squats may not notice anything now, or next week, or next year, but it will catch up to you one day. If you want to keep lifting without complications, take a second thought to range of motion. Im sure if you talk to seasoned lifters they'll be more than interested in talking to you about their bullheaded injuries.