This wiki gets its genres from iTunes. It doesn't use subgenres. ITunes isn't a perfect source, but it has a definite list of genres and eliminates the need for most judgment calls.
To find the genre for a song, go to the iTunes store, type in the name of the song and look for the artists name on the side where it says "Artists and more". Underneath their name is a genre.
I should also mention that the genre of certain artists/songs change every once in a while. For example, Pharrell and his song "Happy" changed from Hip-Hop/Rap to Pop and back to Hip-Hop/Rap again over the past few weeks. I check back with iTunes every week when I put the new week in to see if the genre has changed.
I'll probably add more to this blog later.