The only purpose of having rules of grammar, punctuation and spelling is to make the meaning clear and unambiguous. If I were discussing my recorded music collection, it would be acceptable to abreviate "compact disc" to "CD". When referring to several of them, I would write "CDs". If, however, I am entering text on a form or into a computer that demands block capitals, "CDs" would come out as "CDS" which could easily be misinterpreted. In this case, I would enter "CD'S" even though it is not possessive. I suppose it could be seen as a contraction, with the apostrophe replacing "-isc" but, for consistency, the contraction would have to be C'D'S which is silly. I think that the apostrophe aids clarity and I will continue to use it this way, whatever the pedant thinks.

DC