When a question is asked it is usually tagged according to the problem. After an answer has been selected should it be best practice to edit the tags to reflect the answer?
Generally speaking, if you find yourself in a situation where you're retagging a question due to the answers it got, the question was really tagged wrong to begin with. Tags like bgp or snmp are pretty steady-state. Can you give an example of an answer that you thought had a change in tagability from the question?
It's a good intention, making the topics of answers more visible. However, tagging is for questions, not for answers.
For example, if a question asks about a Juniper firewall issue, but gets a good answer which uses a Cisco ASA instead, we shouldn't add a cisco-asa tag, since the question doesn't deal with it. Even if the OP would accept it and buy an ASA instead.
However, if you see that a tag is really missing to the question, possibly because the answer made the question topic clearer, go ahead and add it.
Another possibility is, that the question could be improved, without changing the specific scope too much of course, then a matching tag could be a good addition. In any case, the tag should match the meaning of the question.
I think if the tags help to refine the categorization of the Q, then by all means. For instance, we have two tags here "wireless" and "wifi". Wireless description used to say it was for WiFi or WLANs, but I adjusted that as it was too narrow for a networking site.
I have been tempted to go back through the "wireless" tagged Q's to add "wifi" to those that are 802.11 related, as this would more accurately reflect the Q and help with future searches.
Edit: I decided to do just that for several that are clearly related to 802.11 wireless.