I was unsure, so I searched it before posting.

Style guides and usage books don't agree.
The Chicago Manual of Style and others recommend dos and don'ts.

The Associated Press and others recommend do's and don'ts.

Eats, Shoots & Leaves recommends do's and don't's.
What Should You Do? Unless your editor wishes otherwise, if you write books, spell it dos and don'ts; and if you write for newspapers, magazines, or the Web, spell it do's and don'ts. If you're writing for yourself, spell it any way you want. Just be consistent.