I use tags mostly to cover the broad strokes of the work: stuff like genre, kinds of relationships, rating, presence of canon characters, fakémon vs no fakémon, that kind of stuff. Other than that I mostly tag the characters and their species, for people who want to read specifically works about Nidorans (they're cute aren't they) and, for the one time I had to do it, CW about stuff like major character death.
But I don't do any of the "tumblrina" style tags such as "conversation in the tags" and the like.