When you relax your glutes, it does put a strain on your knee but your not supposed to relax your glutes.
As far as the knees going forward it really depends on the placement of the bar on your shoulders. With a higher bar, closer to olympic you can afford to let your knees go past your toes more than with a lower placement powerlifting style.
Problem is many people either lift or put plates under their heels. When doing an olympic style squat you shouldn't do that because it will cause you to lean forward and the bar's center of gravity moves forward past your feet. If your going to let you knees past your toes it should be because you have flexibility in your ankles and your back should not round and stay more upright then a powerlifting squat.