There is often enough that a feature-request is a good one and it does get implemented. Prior to the implementation of that feature, there isn't necessarily a tag for that feature (by virtue of the fact that the feature doesn't exist yet). Is it proper, then, that once the feature is implemented, that if necessary the original request be retagged with the tag for that feature? This is primarily for new implemented features which have enough substance to produce new tags. I have two example cases where this could turn up as a concern.
[linked-questions] was added to represent the new "Linked" section. There was no specific request that addressed this, and in fact, Jeff announced it on four separate occasions. In this case, the original "requests" were not necessarily directly about the Linked section, it merely was the implemented solution that was used.
[auto-comments] is a tag I'm adding (if it didn't already exist, which I'd be surprised for this) in light of the recent rise of questions on the subject. While looking for candidates to retag, I stumbled across the original request, which is explicitly requesting for auto-comments. In this situation, the original request is indeed about auto-comments, the tag simply didn't exist when the post was first made.
So, what do people think about this? For my own take, adding the tag helps people find the original request for certain mechanics, but in some cases it may seem like you're tagging the answer instead of the question (which feels awkward to me).