I think it's very important to set the customers expectations appropriately upon noticing potential problem areas. If the problem areas fibers are matted or damaged, I like to get down low and show the customer the difference between undamaged areas, by running your hand across it to show the difference between undamaged and damaged areas. I don't think it can be taken for granted that the customer actually understands how fibers can be damaged, and that some things are not 100 percent correctable. We cleaners "see" these areas differently, and I find most people don't give it much conscious attention until pointed out and explained correctly.
Assuming you are confident that you have cleaned it to the best of your ability, I know some guys like to mist these areas with 40 vol, especially on lighter carpets.
There is a product made specifically to improve the appearance of these areas that has gotten good reviews, but I have not tried myself, is Pros Choice Dinge Away.
http://www.proschoice.com/html/dinge_away.html