I think you'd have to ask yourself for each possible tag if it improves the site or not. A tag improves the site if it helps users and visitor find questions. A tag doesn't improve the site if it harms findability.
Whether it does, especially with events that are likely to end soon (trump-impeachment probably won't continue for decades), mostly depends on how many questions we have that fit the tag. The more question a tag has, the more likely it's helpful to users.
The problem with that is of course that you won't know how many questions there will be when creating the tag.
So, a simple trick might be to wait with creating event-based tags until there are enough questions for it. If we had some threshold value
k, then we could wait until there are
k relevant questions and then create the tag.
The reasoning is that when there are
k relevant questions then there's enough interest in the community to provide knowledge on the event and it's useful to find other questions about the same event.