I've been playing around with Adobe's Audition 3 and found that by converting files to "MPEG Layer-3 Pro, VBR(100), ~117 Kbps (Stero); 44100 Hz, 16-bit, Stereo" (that's a direct quote from the file info provided by Audition on a converted file) I can reduce the size of mp3 files by one third without loosing any sound quality.
When the same file is brouight up in Mp3tag the file info is different -
Codec - MPEG 2 Layer III
Bitrate - 144 kBt/s
Frequency - 22050
This is also the info provided in iTunes.
It does not affect the other information.