Phew...
FYI, the 2 custom sorting lists can be appended to without your knowledge when importing files where sorting tags are present: ArtistSort, AlbumArtistSort, AlbumSort.
The assumption with any and all tags present in a file that you import is that they are there intentionally. If you prefer not to have the sort tags used, you can, as you suggest, delete them prior to scanning them into MB. Or just disable the checkbox for using the custom sort values in the Preferences>Sorting/Grouping tab.
