Even if we believed Microsoft marketing, 64kbps WMA would be equivalent to 128kbps CBR MP3, where even I can sometimes hear artifacts without listening too closely.

OTOH, hearing is beliveing, so why don't you pick several chalenging tracks, rip them, encode them, and them compare WAVs to WMAs (through a good set of headphones). Try various stuff (vocals, choirs, synths, full orchestra, lots of percussions...) The usual procedure is to find encoding quality level where one cannot hear any difference (or in the worst case decides he can live with it), and then use one higher.