Chambers suggests that 's can also be used to form the plural for lower-case letters and numbers ... m's or 3's.
Also, it specifically gives "Do's and Don'ts".
The whole point of punctuation surely is to remove ambiguity and avoid misunderstanding. How to apply it might not always be best served by the application of fixed rules, particulaly in the Enlish language.
"Raining cat's and dog's" is clearly wrong becuase you could think "Cat's and dog's whats??".
But "Do's and don'ts" or even "Do's and Dont's" are much clearer (and better looking) then "Dos and Don'ts".
"Dos" not only looks like a victim of txt spk, it could mean a pc operating system or, a deliberate use of a foreign word.
I wish I knew.