I think a lot of webmasters (including us) put up a 2257 exemption notice if there's content that's even slightly questionable. Our boysfeetclub.com site has no explicit content and almost no nudity so it's clearly exempt, but we maintain 2257 notices nonetheless.

Much better to overcomply and be safe than to misjudge and try to argue with the justice department about why your site doesn't need labeling.