I think, and I said this over at Vera's, warning is really the wrong word - labels like slash or het, or even listing the exciting kinks contained within, it should be more like a statement of genre, or an ingredients list. "Warning" presupposes a lot of bad stuff; plenty of people might be actively searching for kink X or slash or het. Certainly that's how I'm most likely to be found using the "Warning" header. The only genuine warnings I feel the need to note would be the inclusion of something potentially triggering like rape, or strong violence.