In "Eats, Shoots & Leaves", the grammar book by Lynne Truss, she states that apostrophes are used to indicate the plurals of letters and the plurals of words:

How many f's are there in Fulham?
do's and don't's
but's and and's

I've done some googling and noticed that Telegraph newspaper journalists have used the expression do's and don't's and also dos and don'ts.

Personally I go with Lynne Truss - as dos is confusing for the reader.