Trust me, if you need to compress the rear brake calipers, you will need the tool.
They won't compress under pressure...only by screwing them back with the tool.

 

OK if you purchased good quality rotors you should have no need to worry about them needing to be turned. However I know ofa fewbrands that actually need to be turned right out of the box that's how crappy they are.As for why Bill did not need to compress his pistons. He reused his stock pads. I also did this since they are practically brand new but on the front i did have to compress the dual pistons slightly. You will need the tool posted above for the rears though a "C" Clamp will NOT work. All i had to do with the rear though was simply release the parking brake that gave enough clearance to re install.

Richard