I don't think it should be offensive to point out trends that are plain to see exist. As an example, since we have a "history of metal" thread - how many black musicians are mentioned in this thread? Very few - possibly only two I would think, which is Jimi Hendrix and Phil Lynott. Metal as a genre of music today is largely defined by white people like the members of Metallica and Black Sabbath. I'm not saying metal excludes black people, but it's not as much part of their cultural heritage. Look at the blues and it's a different story.
Observing a difference isn't the same as saying that difference is right or wrong, so I think you're in the moral clear
I think there are some things you forgot when it comes to stereotypes, though - what about nationality and age? Those are things I might draw conclusions from before actually knowing. There are some kinds of music that I think of as distinctly american, for example the commercial punk sounds of NOFX, Blink 182 and Good Charlotte and the emo power rock from bands like Nickelback, Creed and perhaps related, Evanescense.
By nationality, I stereotype finnish people as well. The stereotypical fin of about my age (late 20s) listens to metal, often cheesy, and their favourite bands lists is going to include a lot of bands from his or her home country. I've met quite a few fins who more or less fit this stereotype as well so in my experience it's been reasonably accurate
It's not a bad stereotype, though, I'm fairly compatible.