Ass to grass.. The weakest point in that range of motion would be at parallel. Go lower for full exercise of range of motion.

Keep shoulders in front.. Ass in back. not fully compressed and not fully locked on either end of the motion. don't let your joints take the strain of the weight. That's what the muscles are for.

Same as the bench press.. Don't lock your elbows after raising the weight.
Bad for your elbows. Always keep them slightly bent.