The problem with weathering, and I think this is the answer to a debate which is older than any one here is: Once you've whethered on part, you can't stop. Next you have to add tyre wear, scoring on the brake discs, bugs, oil stains, dirt, worn seats, etc etc.
If your going to add realism to a car, to make it look a little used, then you need to make all of it used.
But if your going for straight off the show room floor look, then you don't want to add wear any where.
If your going to Weather parts, you have to weather the whole car to keep it all in balance.