This question has been discussed on other stackexchange sites as well but I feel this is a good time to ask it and get a basic understanding of where we are at. So here we go. When asking questions the OP sets tags. These tags then reflects the question as the Original Post sees it. We can edit the tags list if there is an obvious omission. When answers arrive it may become evident that the question may not completely reflect what the answers reveal and so when reading the Q-A combination new tags may be appropriate.
As I see it, there is an advantage to have tags reflect the entire Q-A post rather than just the question. This is because when looking for similar information by using a tag, the information may not necessarily be in the question but rather in the answer. A good example from TeX.sx is when people ask how to solve a graphics problem. Usually there are two basically incompatible avenues to go down (pgf-tikz and pstricks) and the question may not ask for either of them. Hence answers concerning these solutions may go untagged with respect to these major directions.
Some say you can simply do a free form search. This is of course true but then we would not need the tags in the first place. So the question from me is tags for question content only or tags for combined Q-A- content. I look forward to see where this leads.