If you missed the Mastered for iTunes tools that Apple released at the beginning of this year you should take a look and grab the AURoundTripAAC Audio Unit if you want to test if you can hear the difference between your source material and the resulting encoded file.
My results were about what I expected. I did VERY well hearing the difference at low bit rates.
16kpbs is like shooting fish in a barrel. If I bump it up to 96 things get a little tricker for me:
So I think 256kpbs AAC files will be just fine for my listening.