they're used if needed, but there's some amount of adjustment possible in just the standard factory mountings such as the door hinge, which on some series have slightly oval holes - you'll also find the rad core support mounting can affect all of the front sheet metal fit including how it meets the cab
in the factory they had the 'luxury' of setting aside a piece that didn't easily fit well and using one that did, if you're swapping in parts from another truck or if you've done much metal work, you might need shims that the factory didn't
Bill
