ATG Squats are great to prevent inbalances in your squats. But you can also make use of partials with > your 1-RM to get used to the weight (negatives, lockouts, etc). For safety reasons I would recommend using a squat or power rack whenever you are doing more than just technique work for your back squats. Front Squats can be safely done at any weight provided you can clean it and bail the lift if necessarily. If you look at the vid where Kaz injured his leg when squatting, he was using squat stands and multiple spotters. The injury could have been much worse if he didn't know how to bail from a failed lift. Also, he admitted that his set-up was off. So make sure to get your set-up ritual down to a T. So basically, I would recommend you practice technique as well as bailing and controlled "failures" where you lower the bar to the pins/rack when you fail a lift (whether you train to failure or not I believe this is very important). Sorry I'm verbose, I need to work on that.