Possible Duplicate:
Should I retag a question with a tag that is based on the answer and not the question?
Retagging questions in terms of possible answer technologies
Retagging questions with the answer?
There is a question about problem X. Most of answers mention that it can be solved with technology A and show how.
Should that question be tagged with A that isn't mentioned in question itself at all?
Ais in accepted answer then such tagging makes good sense generally. Otherwise, I would say deciding on this would better be left to someone with expert knowledge (high rep) in technology A - to someone able do judge if the question is indeed relevant for tag to apply