One of the basic principles of any classification system is helping the user find what they want. If you're the only (or principle) user of the collection, you can use any system you like that works for you. Looking at Dewey, or Library of Congress, or other systems might give you inspiration, of course.
Personally, while I'm happy to use those systems in my professional life, they don't fit my personal library at all - I have large numbers of books in areas both LOC and Dewey handle quite badly (religion that isn't Christianity, mythology, the use of technology tools). So I sort these sections in an order that makes sense for me. And within fiction, I prefer to browse and therefore sort by very specific sub-genre (urban fantasy, historical mystery, etc.)
The other part has to do with the practical realities of space use: if I have 5 shelves worth of a general subject, I'm going to store those somewhere I have space for those five shelves, even if logically, they might fit better between one topic and another. Or I might choose to store those books in the room where I use them a lot. Home use has considerations a public space doesn't.
One informal option for managing a collection is using a catalog (I use LibraryThing, but there are plenty of others) that will let you tag with different topics - that way, it's easy (once I set the tags) to browse a list of items about a particular subject even if they might be shelved in varied places for other reasons (size, where I'm most likely to want to use them.)