Rule 11:

Exception:
Use apostrophes with capital letters [sic -- the first example uses a lowercase letter] and numbers when the meaning would be unclear otherwise.

Examples:
Please dot your i's.
You don't mean is.

If you were looking at the link I posted before editing my comment, search for "tired" and "DO use the apostrophe to form the plural".

My 1992 Little, Brown Handbook says:

Use an apostrophe plus -s to form the plurals of letters, numbers, and words named as words.

That sentence has too many but's.

Remember to dot your i's and cross your t's, or your readers may not be able to distinguish them from e's and l's.

At the end of each chapter the author had mysteriously written two 3's and two &'s.

[...]

Exception: References to the years in a decade are not underlined [italicized] and often omit the apostrophe. Thus either 1960's or 1960s is acceptable as long as usage is consistent.