What's the rule for apostrophes when it comes to abbreviations?

You should not use apostrophes. One of the purposes of an apostrophe in the English language is to replace missing letters in a contraction. Therefore, because there are no missing letters in an acronym, an apostrophe should not be used.

The apostrophe has three uses:

1) to form possessives of nouns

2) to show the omission of letters

3) to indicate certain plurals of lowercase letters.

Apostrophes are NOT used for possessive pronouns or for noun plurals, including acronyms.

UFOs and CDs are the plurals. UFOs' and CDs' are plural and possessive.

I saw the one UFO in the sky. (singular)

I saw the UFO's light in the sky. ( singular and possessive)

I saw 10 UFOs in the sky. (plural)

I saw 10 UFOs' lights in the sky. (plural and possessive)

As you can see, abbreviations follow the same rules as other nouns.

In plural form apostrophes are not used.