It depends on YOUR needs.
I don't care about 8 million genres or tags. I know what most of the books I have are. I know the authors and what they write. So I just need a grouping for some one off or small numbers of books.
Whatever works FOR YOU. I was just presenting a way of doing it that works for me.
Also, I use these to create collections on my Sony. I don't need a Fiction collection of 1400 books, and a Scifi collection for 600 books, all of which are fiction. If it not Non-Fiction, it must be Fiction.