I used to dump my high bar squats all the time at the end of my set (onto the floor as the gym had bumpers). I've only ever missed a squat rep once in my life - it was a low bar squat and even having the safeties set correctly it was not something I'd describe as pleasant. You run the risk of the bar hitting the back of your head and knocking you out also. I would only ever let people I know spot me as well, I felt very secure with the guys from PTC spotting me. If I go to nationals I would probably get Zach, Sticky or Ricky to spot me as I know them to be quality people. Who else would you trust your life with?