There is more to it than the remaining rotor thickness, although that is a good place to start. You need to evaluate the surface finish of the rotors, look for hot spotting, and so on. Many times it is possible to reuse rotors, whether front or rear. Many of us don't do so, because it isn't worth our time to have to go back in and do the brakes again if we get pad noise after not replacing the rotors. There isn't a lot of meat on BMW rotors (likely due to a decision to reduce unsprung weight). You can't turn them, which used to be standard practice. If they get too thin, you will have problems, probably more to do with increased brake fade than with warping.

Many shops (including dealers) replace rotors as standard practice due to their desire to reduce service warranty redo whereby customers come back and ask the shop to redo the brakes because of squealing post pad change.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue