Per my comment, I'm going to say "Never" or at the very least "Not for a long time".
For over a month now, the 'tagging team' have been pretty active in destroying lesser used tags, adding all manner of synonyms and untagging questions. What's been confusing me is why?
Whilst it's quite straightforward to see the damage caused (repeatedly flooding the front page with dozens of minor edits) where I'm struggling is trying to see what the benefit actually is.
I'd like you to elect a spokesperson who can explain (using graphs and stats) how all this retagging has helped the site.
Until then, I think you should stop.