I try to use a high bit rate when ripping to mp3, no less than 192kbs. For classical and more serious listening I use 256kbs. A significant amount of testing has been done at different mp3 bit rates. At 256kbs it is impossible to tell whether you are playing an mp3 or the orginal cd file.
I realize many people will claim they can hear a difference. I'm fortunate to not be one of those people.