We live in a world of change and what was correct yesterday may not be correct today. I don't really think it matters how you spell do's and don'ts as long as the message is received correctly by the reader. It's like reading the IRS Code. There is usually an exception to the rule somewhere. Maybe it used to be "dos" until we had to deal with DOS. If you use do's just to help the reader understand that there is a list of what to do, then it's correct. "Does eat oats and mares eat oats and little lambs eat ivy." So the plural of doe is does and the plural of do is dos. Or is it do's?