Practically speaking, you are right. However, as with all good rules, this one has a couple of exceptions, both in the somewhat unusual case where you are talking about letters as letters or words as words. The following information comes from Sections 6.77 and 6.82 of the 14th edition of The Chicago Manual of Style.

Letters as Letters

Letters and combinations thereof are properly set in italics and the plurals formed by adding s or es in Roman type. Thus:

the letter q
He signed the document with an X.
There are four is in Mississippi.

However, in some proverbial expressions the distinction is ignored, and in that case the plural is formed by adding an apostrophe and s. Thus:

Mind your p's and q's.
Dot your i's and cross your t's.

Words as Words

The plurals of italicized words are formed as are the plurals of italicized letters, by by adding s or es in Roman type. However, when it is required or preferable to enclose the word in quotation marks (instead of italicizing it), the tidiest way to show the plural is to add an apostrophe and an s inside the closing quotation mark, thus:

Tim had enough of her "maybe's."
How many "not applicable's" did Sidney enter?

I sincerely hope that this is more than anybody here wanted to know about this subject. :)

-- Russ