Depends on what your trying to acheive. I do back squats with my shins perpendicular to the floor so its almost physically impossible to do ATF. A bodybuilder squat where your knees pass your feet you could go ATF. The only guy Ive seen squat ass to the ground without knees passing feet is Kirk Kawalski. I still dont understand how he does this. He must have a unique structure to his body to allow this. Front squats I do ATF because of the mechanics of the lift.