Your knees should not track beyond your toes. Yes this is dangerous as it causes undue stress on your knees. If your knees are tracking beyond your toes:Will a full ass to grass squat, with my knees no further forward than my toes (think a baby in a diaper that just stops and squats down to grab a cheerio or something) wreck my knees?
1.) you are not pushing your ass *back*
2.) you are not pushing your knees outwards while lowering and raising the weight
with that in mind you're probably safe if you aren't doing that while retaining curvature in your spine
Ideally you want to get below parallel on your depth because that's where the real gains to strength stem from; that's different than ass to grass but trust me once you get substantial weight anything approaching parallel to the ground feels like you are actually 6 inches lower. A great way to learn would be to film yourself doing them and upload a video to youtube for specific feedback.
- your back should remain straight but not vertical/perpendicular to the ground - think about sticking your ass out behind you as much as possible while lowering the weight - that will help keep your knees from tracking out over your toes