Quote from Brass:
I only went parallel when I did squats, and I set the safety bars just below the parallel point of the movement. It came in quite handy a few times when I overestimated my ability to do an additional rep. As for ATG, I can only imagine the bars would need to be set at around, or very slightly lower than, ATG so that you can maneuver your way out from under the bar if you find yourself at the bottom but can't get up. However, I do admit that for ATG, it seems somewhat riskier than for parallel, and I'm really not the guy to ask. The "spot" portion would be where the bar doesn't fully have its way with you at failure.
As I noted earlier, I stopped doing squats about 7 years ago, in my mid 40s. I figured if there was one exercise that could do me serious harm if I overestimated myself and wasn't properly focused even momentarily, it was the barbell squat. And so for me, "better safe than sorry" was saying goodbye to barbell squats, because I like to go all out but didn't feel that squats were a good fit for that as I got older.
As an aside, the pistol squats that I can't seem to shut up about are ATG, and I have gone to full failure on the last rep of the last set, where all I had to do was descend back to the bottom of the movement and release the dumbbells. Not much drama there, but as hard an upper leg movement as any I've ever done.