Thanx for explaining how Mp3tag handles customization. I need to find some time to read the documentation.
When I need to edit most of the tags in a song I use TagScanner, Mp3tag, or Mp3 Tag Studio. I need to standardize on one of them, but I find each more useful than the others for certain tagging chores. Mp3 Tag Studio is particularly useful for editing stuff (flags) that are not saved in a tag. Anyway, I use them to mostly automate the process (pulling info from internet sources) but I often disagree with the genre provided. It's not so much that the provided genre is wrong. It's most often more exact than I need it to be.
When I need to edit 1 or 2 tags in a song I use the tag editor in Winamp. I usually edit the v2 tags and copy them to the v1 tags. Sometimes the v2 genre tag is changed when copied to v1. For example, "slow jam" is changed to "slow rock", clearly not close to the same thing. This is the kind of sync problem I was referring to. I can live with the kind of change in your example, such as "hip-hop" to "hip hop".
So it would be useful to be able to edit the default list of genres and not just add new strings to the end of it. While time consuming, being able to 'fix' this stuff manually is better than nothing.