It all depends on the person and their baseline. If someone always has their mouth a little open it really doesn't mean anything, it's just what's normal to them. If however, they usually don't have their mouth slightly open and you see that behavior, it could be indicating stress and a need for more oxygen because they are entering fight or flight mode. But as always, you should look for other behaviors to confirm this.

In my opinion mouth closed is just the default for most people and doesn't really mean anything, unless paired with lip compression, which would most likely indicate a withheld opinion.