Folklore from many years talking about this with dealers says that the rotors are almost always replaced- because the BMW specs do not allow for very much "turning" and therefore not much room to take metal off. Apparently the rotors are thin to begin with so they cool faster...

now- for the part that will start a range war....

I had a 735i once, needed brakes, and I ended up at Midas, where they installed 4 rotors, pads, sensors etc, labor and LIFETIME Replacement guarantee, seems like 3 or 400 bucks... Car stopped fine, less squeaking, and far less dust.... FYI