The usage of the marvel-cinematic-universe tag seems quite a bit inconsistent at the moment. While it is clear that it should be used for any question pertaining to the general cross-movie intricacies of the Marvel Cinematic Universe, it seems largely unclear if it should also be used for individual movies. There are many questions with that tag that are not about the MCU in general but simply about the story of individual movies. Likewise are there many questions about individual movie that don't have the tag. So how should this tag be used?
We already have some larger movie series tags which are (supposed to be) used for each and every question about a movie of that series, in addition to individual movie tags (if necessary), like star-wars, lord-of-the-rings, pirates-of-the-caribbean, harry-potter, ...
However, the individual movies there seem to have a slightly more tight relation and similarity. They are much more "movie series" than "movie franchises" (if there even is a difference between the two terms). This is especially notable in their titles, which more often than not include the series as part of the title (of the form Series: Movie). This makes using the series tag also a practical matter of avoiding overly long tag names. This isn't the case for the MCU, likewise don't we have tags for each and every remotely connected movie franchise (let alone tag each and every question with those). Neither are we supposed to use too much tags to describe the movie the question is about, since tags are supposed to describe what the actual question is about. In the same way we don't tag questions about individual movies with genre or director tags if the questions are not about the genre or director specifically.
On a merely practical note, using it for each and every MCU movie would of course mean we'd have to retag each and every existing question that doesn't have it yet, and there are quite a few of them. But even more so, we would have to enforce its usage on each and every question about an MCU movie that comes up in the future. That is something new users are much less likely to do correctly if the series name is not an obvious part of the movie title and they might not even be aware of the existence of a broader franchise those movies are part of.
So the primary question here is, is the Marvel Cinematic Universe contained and connected enough that questions about its individual movies have the obligation to be tagged with marvel-cinematic-universe and would be incomplete without it, or if it is not a sufficiently tight movie series to warrant polluting the limited 5 tag slots with what amounts to just a classification of the movie and not the actual question itself.
(This discussion of course also would have the potential for setting a precedent for other less tightly coupled movie franchises to warrant tags that are used on each and every question. Something to bear in mind.)