I suggest we introduce tags clearly showing that a document class is concerned.
letter-class could be distinguished from questions regarding letters as such.
A first glance at the tag shows that a class is meant. We could omit a documentclass tag usually.
If I would get some positive comments and the question, let's say, 10 upvotes, I would create the [tag:*-class] tags plus some synonyms. For instance, beamer would be a good synonym for beamer-class but slides perhaps not for slides-class.