BMW does not recommend changing rotors with the pads, they frown on turning rotors and should the rotors be worn or grooved to the point that they would need turning, than BMW recommends replacing them. If your rotors are above the minimum thickness, which they should be assuming this is the first time you have replaced the pads they should not need replacing.
If your rears are shot than your fronts should be as well as they do most of the braking! Have you already replaced the fronts?