Recently (half year ago) I switched from Army Modeling to car modeling just because I was tired of heaving to weather everything I made. But than again a Tiger I tank that isn't weathered looks like sh**
Personally I think the opposite counts for cars (even rallycars), I think they should be shiny as a mirror unless you want to use them in a diorama off course.
If you do want to weather and need some additional articles or how-to's, maybe have a look here:
http://fp.docdio.f9.co.uk/page/articles1.htm
http://www.missing-lynx.com/dg.htm
Allthough it's mostly about army vehicles and diorama's the techniques can also be applied for cars.
Grtz,
Jan