The best practice is to omit apostrophes in plurals unless necessary to clarify meaning.

A's instead of As might be appropriate because "as" is a word, but I don't see how anyone could mistake "2s" for a word instead of a plural of 2. I have noticed that the apostrophe is rarer than it was, I think because of the increased ability to alter fonts and text size in lieu of punctuation to clarify meaning. So too, have periods disappeared in abbreviations.