I think the only time I marked "viewer discretion advised" was when one of my icon batches had one with artistic nudity, so I ticked the setting so that people could avoid that if they wanted. Otherwise I figure that if I've got anything worth warning about I'll cut and note why in the cut text.

(Also, I read DW for a long time before I got an account, and the constant 18+ warnings to click through were kind of annoying.)