Here's a local body art place I blogged about
: Do's & Tattoos. I'm not exactly sure what the rule here would be. Technically, to indicate the omitted letters, it would be 'Dos. But I think there's a rule that says if the result is unreadable, you can put an apostrophe in the plural to make it read better. Without the apostrophe, people 'round here would read it as "dos"; i.e., the number two. So, it would be . . .
'Do's.
Here's a link to the blog post, if you're interested (but no biggie):
http://piercedconsumer.com/2009/11/03/dos-tattoos/