I go Parellel or a inch or 2 below. I use boxes, so, thats where I stop. Going down too far can put added stress on the knees, cause lumbar strain and they cause Hip joint problems.

Check out some of the pics of low ass too the floor squats. Olympic squatters
http://users.pandora.be/tom.goegebuer/Gallery_gen.htm

I doubt many people here can go that low.

I squat barefoot cause Im at my house, many squatters and WSB recomend chucks for foot gear. Ill be picking up a pair soon, they are being discontinued.