I allways look up the artist on allmusic.com and READ about the genre.
After many hours of studying I can say I have a pretty good overview of the genres.
I found it best to divide the genre into genre and sub genre.
My genres areGenre:
Blues, Country & Western, Electronica, Hip Hop & Rap, Jazz, Others (comedy, mix etc...), Pop, R&B, Reggae, Rock, Techno & Dance, World
Sub Genre:
Acid Jazz, Alternative Hip Hop, Alternative Metal, Alternative Rock, Ambient, Big Beat, Black Metal, Blues, Britpop, Celebrity Pop, Chillout, Classic Jazz, Classic Rock, Club/Dance, Comedy, Comedy Rock, Country Pop, Dance Pop, Dancehall, Disco, DJ Mix, DownTempo, Easy Listening, Electro-Funk, Electropop, EuroDance, Folk-Rock, Folklore, Funk, Funk-Rock, G-Funk, Glam Rock, Goa Trance, Goth Rock/Metal, Grunge, Hard Rock, Hardcore Hip Hop, Hardcore Rap, Heavy Metal, Hip Hop, Hip Hop Soul, Hit Collections, House, Indie Dance, Indie Rock, Industrial, Industrial Metal, Industrial Rap Metal, Industrial Rock, Mainstream R&B, Metal, Metalcore, Miami Bass, Minneapolis Sound, Modern Rock, Neo Soul, New Jack Swing, Norwegian Hip Hop, Nu Metal, Oldschool Hip Hip, Oldschool R&B, Pop Rock, Post-Rock, Post-Grunge, Prog Rock, Prog Trance, Punk Rock, R&B Pop, R&B Soul, Rap, Rapcore, Reggae, Rockabilly, Soft Rock, Soul, Soundtracks, Southern Rock, Stoner Rock, Tech House, Techno, Trance, Tribal, Trip Hop, Vocal Pop.
Rock is a huge part of my sub genre. 34 of 86 sub genre is part of rock.
With 13 000 tracks, this was a pretty extensive job. But I simply LOVE it now.
I can get auto generated smartlists with the desired genre/subgenre mixed with BPM and intensity.
And with all my tracks rated as well, my life of music is just sweeet
The most important thing of genre is not to place the artist "correct". It is for you to know the genre
(a little), and place the artists/albums/songs acording to where you feel it belongs.
After all it's your library.
There are no right answers when it comes to genres