Using an interest tag on a Facebook post can reduce the reach of the post. However, it can also ensure the post is seen by more people who will appreciate and / or engage with the post.
My advice is that you research a tag prior to using it. Get a feel for its value in terms of reach. If you gauge sufficient appeal, use it.
I used the tag teacher recently and got a post seen by people who may have otherwise missed it. The actual reach of the post was lower than usual yet the practical engagement was higher.
Tagging interests takes time. The rewards can be valuable.