Rocking back is not recommend it probably gets you off balance and is a substitute for actually engaging your lower half properly, which is usually described as bent knees in sprinters position
look at the stills below, Amanda's high back swing is not needed but focus on her legs.
Depend on what you mean by rocking back. Does she need to pull her shoulders back? No. But there should be some shift backwards of her center of gravity to overcome inertia so that when she's ready to go forward she's not wasting energy getting her body going. If you watch this video of Amanda Scarborough you will see her shift her weight back a bit before she goes forward, despite the Softball Power Drive really restricting that type of motion.
"The rearward slide and center of the drive foot is absolutely critical. It shows you that she transferred ALL her weight backwards onto the stride foot and MOVED backwards. As they progress with this drill, most pitchers need to be reminded not to rush or skip this super important rearward move. This rearward move creates more distance to travel through prior to the drive foot ‘pushing’… which is what creates momentum (like a walkthrough). As these mechanics become her regular pitch, be sure this move happens."
Think of it like hitting or even throwing overhand. There has to be some rearward motion/load first if you're going to maximize power.
I use a drill where I have pitchers start in a sprinter's position, about as far forward as they can get without falling over. It's to show them how their hips should be getting out over the front foot instead of sitting back over the back foot when they launch. The drill has been pretty effective. Even then, though, I recommend they rock back just a little bit to help themselves get going.
That is what elite pitchers do. Take a look at Monica Abbott. Despite the illegality of the gym step you can't do that if you don't transfer the weight back to the back leg.