grammatical number - Is “ ’s ” ever correct for pluralization?

A relatively modern dictionary (I don’t know which one, because we’ve cut out the pages and used them as wallpaper in our bathroom, but I know it’s less than 20 years old) indicates that R’s is one correct pluralization of R, as is Rs, but whichever dictionary this is, it’s kind of a no-name brand, so I’m not sure I trust it.

I’ve always wondered what the best way was to pluralize single letters or numerals, like 2’s or 2s. What’s correct?