I personally do not advocate whacking rats as a method of pre-killing.
I feed live to all of mine, never stunned, never pre-killed. Ball pythons are amazingly efficient hunters, and are quite able to take care of the killing themselves with extremely low risk of injury.
I have ~ 40 ball pythons. Let's just say conservatively that I feed 1 rat a week to every one of them (some eat more than one). I've probably fed over 8300 live prey items to my snakes in the time I've kept them. In all those feedings, I've had probably 5 minor scratches. That makes my personal risk of injury .060%. For me, a .060% likelyhood of injury is very low risk and is not compelling enough of a reason for me to feel the need to stun my prey before it's offered to my snakes.