@whuber recently gave some reason as to why we should avoid trivial edits to questions/answers:
Users are awarded badges for making lots of edits, editing old posts, etc. Do you want to encourage abusers who make dozens or hundreds of trivial edits in pursuit of a silver or gold star?
Any edit opens a post up to re-voting: existing votes can freely be removed by the people who made them.
Sometimes even a tiny edit materially changes the correctness or meaning of a post. When that happens, trouble brews.
Edited posts become "active" and appear in the list of questions.
With the remark that
Because question titles appear everywhere, a good case could be made for even tiny cosmetic edits to titles [...]
I think that retagging recent questions is very similar to editing titles of recent questions: the purpose is to clarify things and make the question easier to find in the future.
But what about retagging old posts? On the one hand, it makes the questions easier to find. On the other hand, we don't want to be flooded by old questions resurfacing because someone added a tag to them.
What is the best practice for this? Only retag questions without (good) answers? Only retag a question if you've made a substantial other edit to it?