If I can't hear the difference, I guess my dilemma goes to Mb9061's point - do I run the risk of re-ripping in the future?
I'd definitely support Mb9061 in his contention that you'd be mighty disappointed in the future to have to rip an entire library all over again. You can always down-rez a rip to a lower rez, but never up-rez (though this can be accomplished, it will not yield better sound IMO and may introduce distortions and or artifacts). Why not rip to a hard drive (cheap) and then down-rez the entire library to a separate folder for use on your iTouch? All that said, if you cannot hear the difference in the files via good headphones then I somehow doubt you will hear the difference on your system (though it is not impossible). It would depend on how resolving a system you have. Try a few comparisons there. Of course that would not take into consideration any future upgrades to that system which may resolve differences to a greater extent.