Some cars with staggered diameters and traction control will have problems if they run a square setup (G35 coupes were the most notable, some cars would be immobilized by the traction control if front and rear wheels were switched).
Based on this thread:
it seems to be OK so faron the M3.
The person you spoke with was probably being conservative, because there is nothing official yet that says "square setup with equal diameters is OK". Edit: When I said nothing official, I meant *nothing official on the Tire Rack database for the M3*. Sorry for any confusion