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Paul SMy understanding is that if it is truly rubbed off (like with a coin), then it will affect the grade, although I am unclear by how much value wise. There is a difference between that and some that have sort of partly "worn" off, for reasons such as being stored unprotected next to other cards. Those kind of look like the white has sort of faded a bit and you can see the print underneath begin to emerge -- looks like a coat of primer paint which hasn't completely covered the undercoat. I have several hundred like that (the latter) and if I remember correctly I don't think that affects value very much, if at all, unless you are going for a card that would otherwise grade a PSA 8 or up.
Others on this board might be able to give you a more definitive answer.
p.s. Jason, this must be the set you were talking about in another thread.