We don't do production studio tags ordinarily, because they don't serve a strong purpose.
Tags pointed out in @Skooba's answer are not tags because they're "production studios".
The 4 that belong are marvel dc dark-horse-comics boom-studios (and also image-comics and some others), because they're specifying a distinct set or sets of canon and/or shared universes. They're franchise tags.
As you can see, this means that the tags aren't being used just because of who created the content. Similarly to how we don't use author tags unless the question is about the author.
So, pixar should only be used on questions about Pixar, not Pixar films. On the same note, disney needs to be heavily cleaned up (it's not used very well or consistently at all right now), and studio-ghibli only really belongs on one question.
To make it clearer, here's a list of major production and publishing houses that make plenty of SFF works but we don't have tags for:
- 20th Century Fox
- Buena Vista
- Warner Bros.
- Sony / Columbia
- New Line Cinema
- Random House
There may be arguments that Disney and Pixar movies have enough hidden references to other films they make, that they're in the same universe, as many fan theories speculate, but there's not really any shared story in those examples, while the franchise tags above have great number of works that often interact with one another.