I scan a lot of cards and one thing I notice, particularly about glossy cards from the 1980s and 1990s is that the cards can yellow badly, especially on the white borders. Here's a recent card I scanned to show the effect.
I also think it would be unethical to actually try to physically clean up the card for selling or for grading.
I typically do not use any Photoshop tricks on older cards, say cards prior to 1980. I wasn't collecting cards then, so I don't have any recollection of what the cards looked like when new. Besides that, the older a card is, the harder it is to clean up.