I really depends a lot on your suspension setup.
For example, I'm running an 18 x 9.5 +22 front and rear on my auto RS4V. With the car at stock height, the wheels stuck out of the gaurds by about 1.5". Mine is lowered to 340mm (from the centre of the wheel to bottom of guard lip) in the rear and it sits very nice and I haven't needed to roll the guards. I have fitted camber arms tohugh, as at that height, the inside of the rims where touching the coilover body.