Up until recently I'd been tagging bikini/lingerie posts as questionable because AdSense's ambiguous wording on what constitutes inappropriate material worried me. But it seems like the site just isn't big enough to warrant their attention since I got what amounted to a form letter when I asked Google directly.
So in conclusion, I think it's probably fine to tag some bikini/lingerie posts as safe, if you don't think it's particularly lascivious. Nudes, topless, cameltoe, and nipples should be questionable however.
Updated