I've always heard that your knees should not extend beyond your toes during squats, or else you'll end up with sore knees.
Why is that, angle of the knees? If the tops of your thighs are at parallel and your knees are that far forward, then the angle at your knees is no different that if you were below parallel with your knees farther back.
Assuming that you really don't want your knees that far forward, then how do you know from rep to rep if they are? The only time that I can see me knees from above is when I'm warming up and I do squats with an empty bar. Otherwise I'm looking forward and slightly up.
Is this knee deal just advice passed down verbally without any solid to evidence to back it up? Kinda reminds me of squatting with your heels elevated by plates or a board. "They" say it's bad for you, but there's plenty of people that do it without problems. I've even read where people advise elevating the heels while doing front squats.
I'm not having any problems with my knees, but I've wondered about this issue before and it's kinda quiet around here right now...