I am beginning to believe that all collectors must have some sort of OCD. I have to think that the majority of people who visit this site do, as they feel the need to case their games and print out corresponding cover art. I'm certainly one of those people, and I thoroughly enjoy it.
That being said, I obviously embrace my OCD and definitely care about cleaning all of my carts. I find it a little therapeutic to be honest, and I am always happy to see a game that was a disaster look like new again.
For me, the label is the selling point. If it is in good condition, the I can usually do anything with the cart. This past weekend I picked up a bunch of NES games, including Taxan's G. I. Joe, which had writing all over it. Luckily, the games were cheap, so I grabbed a couple that were in great condition and just swapped the labels after heating them up with a hair dryer. Now it's like new!