I don't normally go to failure with squats. I dont have a workout partner but usually grab someone on last set of exercise ti make sure I go to failure with heavy weights. However I never get a spot on the squat. It seems difficult to get a squat spot. It also makes me fear going too low as well.

How low is low enough?
Do I go to failure and drop sets on the leg presses and other machine type leg exercises instead of the squat?