Reply #33: The "Chicago Manual of Style"

[Another Bill C.]

says leave out the apostrophe in the case of acronyms, initials, etc. unless it's confusing. Exceptions are initials having more than one period in them, such as M.D.'s or Ph.D.'s.

The "Chicago Manual" also allows an apostrophe in the plural form of single letters, such as x's and y's. This is not a hard and fast rule, though, since the examples given in one section have no apostrophes.

The Associated Press Stylebook agrees with the "Chicago Manual."

I couldn't find a reference to this situation in "Elements of Style."