Is it appropriate to update a question's tags given:
the tag is technically irrelevant to the content of the question itself
BUT is relevant to the answer (well voted or correct, of course)?
Species ID questions will typically ask what kind of XXX something is and tag the question with XXX. However, the species is actually a YYY -- which gets pointed out in the answer -- so should it be tagged with YYY?
This question. It asks about pigments in food, but the answers are rather plant/animal focused. Should a botany or zoology tag be added to the question although it doesn't ask directly about either one?
My opinion is YES because people searching for similar questions later would benefit from coming across questions with tag-relevant answers.