I actually do this. I create LAME VBR MP3s at transparent quality (v2) using Max (
http://sbooth.org/Max/) and then organize then in iTunes.
This may seem like overkill, but VBR MP3s use joint stereo as a standard and the files rarely go above 128k because the encoder realizes that most audiobooks are one voice equally spread between left and right audio channels. This effectively gives me better quality MP3s than I could get with similar AACs.
I can then down-sample these in my audiobook builder projects depending on how much quality they call for. Plus I then have "masters" of my audiobooks later down the road for re-encode. Best of both worlds. It has come in handy on MANY occasions.