I just so happened to change the front brakes on the X5 this weekend.
Since I change my own brakes, I always change the pads and rotors b/c 1) I don't want to worry if the rotor will be past minimum thickness before the second set of pads wear out 2) the work to go back in and change out the rotors and not the pads "to me" is not worth the effort or cost savings.
For background, while under warranty the dealer replaced pads and rotors on the front and pads only in the rear.
I have a
Starrett Disc Brake Micrometer and took a couple of measurements.
These are for Brembo front rotors...
New: 30.40
Min: 28.40
D/S worn: 27.08
P/S worn: 27.12
So for the front when the pads are worn the rotors need to be replaced.