Never during the play.
They may be used for counting VP (if there are cards that grants you with VPs for those tags).
That's incorrect. Face down event tags are never counted, to the end of the game.
Tags of event cards may be used in the moment you play those events. For example there are many events with the space tag, they of course can be paid for with titanium and they get discounts for things like "space station". But later if you get for example the card that gives 1 mc per space tag it wouldn't count your space events because they are face down.
Similarly there are some events with science tags - those count for "mars university" and "olympus conference" who trigger on science tags being played. However they do not count for requirements and they do not count in the end of the game for the scientist award too!
The only other case in which tags would be counted for scoring is with jovian tags. Here the rulebook is a little confusing because it tells you to flip event cards face up to score them before scoring jovians. But luckily there are no jovian events so it doesn't really matter. Even if there were I believe they wouldn't count.