I'd say it's rarely appropriate, but not never. Tags are there to help people find information, but they're limited. If a question yields three answers, adding a tag (or two!) for each answer is going to push us over the limit, so hopefully that's clear as not being appropriate. For three answers, one accepted, adding the tags for the accepted one but not the others risks making it seem even stronger an answer, which may not be fair as it wasn't there to start with. If all three answers refer to the same thing, then it would often seem to be a case where the answer related tag would help people find things later, without confusing. Tagging is there to help, especially for people browsing or searching, so in that case adding a tag relating to all the answers would seem to help, so maybe it is appropriate.
Perhaps another way to think of it is with comments on the question. Someone asks a question, comments are added to clarify something, the OP answers and makes the question clearer, it then feels reasonable to me to edit the tags and add something in that's clear from that. If several people were to instead answer rather than commenting, and clarify it that way, adding the tags then feels very similar to me, so would seem OK. In summary, I'd say it's something that's rarely appropriate, but not never!