One compromise I've heard about is to put a vague warning (or no warning, depending on the author's feelings on how spoilery it is) in the standard header, but then provide a more explicit warning - even if it gives away the plot - in either a footnote or another place entirely, and link to it from the header. So the people for whom it's more important to not be triggered can read the detailed warning, but the people who are more concerned with being spoiled than being triggered don't have to read it.

I'm sort of torn myself, because sometimes warnings are definitely spoilery, but on the other hand, it's horrible to be triggered by fic when you have no idea what to expect. I think that the "double-labeling/warning" idea above at least allows me to make that choice for myself, instead of forcing me to skip the fic entirely to be on the safe side.