Opposite... Too little offset means your wheels are too much "outboard" already... Machining down the mounting surface would do it (tuck them inside the car a little more), dunno about reliability on that though!
The little bump you have to bend in is where the metal fender meets the plastic rear bumper.
With a 43mm offset, the 225 rubber on my 18's doesn't rub at all, and my car is low. Like Brad said, only on the plastic liner inside the fenderwell.
