@hidingplace - Yes, a discretionary rate is also used in the conversion of CDs to MP3s. When MP3s first came into popularity the usual quality was 128kbps which many people didn't consider to be up to the standard of the sound on a CD. Nowadays though the discretionary rate is much higher, with 256kbps or more being the standard. It depends on your setup, but most people wouldn't notice the difference.
The main difference in audio quality is between vinyl records and CDs/MP3s, because a vinyl record is an entirely analog recording.