Fitting custom rims seems to make all 4 tyres the same width, therefore upsetting the balance hugely if the car is supposed to have wider rears. So most road cars and 4WD won't be affected but MR cars and most sports or supersports will likely suffer quite badly. The F40 on which I conducted my testing was truly horrible with custom wheels.
Edit - Bugger off Chris, I thought you were going to bed.