My local Indy said BMW rotors are thin and do not get machined down. New front rotors each time with front pads. Rears you can get away with new rotors every other time when changing pads.
This is what my BMW Indy shop told me. I have not done brakes on my BMW yet... But will within 6 months to a year.
2008 328i E90 Jet Black:6MT:Premium:Sport:Walnut/Black interior:Xenon:MTEC V3 AEs:19" Beyern Mesh:35% Tint:Blacklines:OEM Front Splitters