Sorry for entering the discussion so late, don't know if there is still interest.
I have been thinking about the whole "genre" discussion for a while, coming across the same problem - artists that just won't stay in their genre.
First, a quick recap on music genres.
1. there are only a few real GENRES. Allmusic defines the following:
Avant-Garde
Bluegrass
Blues
Cajun
Celtic
Comedy
Country
Easy Listening
Electronica
Folk
Gospel
Jazz
Latin
New Age
R&B
Rap
Reggae
Rock
Soundtrack
Vocal
World
I take this list as a starting point. Personally, I don't really like it in this exact form. Why?
- One, it is very North America centered. Bluegrass, Gospel, Folk are not really genres of their own,
and in contrast everything outside of the anglosaxon realm is simply shoved into "world".
- Two, their definition of "Rock" is much too broad. I would make separate genres of ROOTS/ROCK & ROLL and POP for example.
But that's another discussion.
2. What comes next are STYLES and there are tons of them. "Rock" alone probably contains over a hundred styles!
With respect to MP3 tagging, the only solution I see would be to define THREE "genre" fields. I would call them "genre", "style" and "artist style".
GENRE is the "father" genre. One of very few.
This allows for example to sort music on the hard disk in a few easily distinguishable directories.
ARTIST STYLE is the OVERALL style of the ARTIST.
I know there are some really difficult cases, but in general you can define a "main" style for an artist.
As an example, I will always consider John Lee Hooker to be "Delta Blues" even if he happens to play "Boogie Woogie" sometimes
STYLE is the individual style of the SONG.
This is very useful if you want to sort out a long list of songs that sound alike, regardless of whether it's that particular artists' speciality.
The big question is now, how should this be implemented?
Media Monkey has "Custom fields" so in theory it would be easy to assign these "sub"-styles to them.
However, what should the GENRE field really contain? The "father" genre or the "style"?
It's a pity that the ID3 people didn't define subgenres when they started the whole thing.
A list of some 99 genres (ID3v1) was either too long (for GENRE) or much too short (for style)
Also, why can't there be a BINDING industry standard for tag frames?
Every MP3 tagging program (including MM

claims to "follow the standard" but I have yet to find two where all ID3v2 frames
would be defined in the same way (and believe me, I've tried LOTS of them!)
Any comments on this topic would be most welcome.
I am a new MM user but have been building up and extensively maintaining a large digital music collection for over 5 years now.
I have been using MMJB and Helium, but am getting fed up with both of them (both too slow and too unstable).
I would also love to hear how other people catalog their music.
One last comment: Am I the only one who HATES the people calling songs "OLDIES"? This is NOT a genre, and the definition changes every few years anyway!