While it's not necessary, an option to alphabetically sort the tags being written to a file would still be helpful.
This would make viewing tags in other programs a lot easier. I notice that things like the four ReplayGain tags in my FLAC files tend to get separated by Mp3tag when I add, remove, and edit other tags.
REPLAYGAIN_ALBUM_GAIN
REPLAYGAIN_ALBUM_PEAK
REPLAYGAIN_TRACK_GAIN
REPLAYGAIN_TRACK_PEAK
Also, for other related tags such as ARTIST and ARTISTSORT, ALBUM and ALBUMSORT, DISC and DISCCOUNT, it's much easier to read the tags when they're next to each other.