I would never add a tag that can be implied by either reading the title or series.
I definitely would not mark books with series:"=Xanth" as tags:"=Xanth", it is nearly as bad as tagging
Centaur Aisle "Centaur Aisle".
What I usually do is delete all tags no matter what, then re-add any tags later, when I decide I need it in order to find books of a similar type.
Character names are somewhat of a pet peeve of mine. They serve no useful purpose, because if you are looking for books about a specific character, you already know what book or series of books you are looking for, and can just search based on that.

But every metadata source (I don't use them anyway, but it is the theory of the thing) includes them.