Splitting by OS makes a lot of sense. Hopefully the tricks that cover all operating systems would be larger.
Splitting by calibre version doesn't make sense to me and may not even be possible retroactively. But I think lableing every trick we know is version specific (e.g., version 4.x and hgiher) is a good idea.
Splitting by calibre functional area makes sense to me, although it's not clear yet what those divisions are beyond editor, reader, etc.