I've been using the --r3mix flag on lame, which gets VBR and all the other bells and whistles you might want. What I like about VBR is that it stretches when it needs to use more bits, and relaxes otherwise. When I'm ripping old 1930's mono recordings, they come in around 112kbps. When I'm ripping current stereo recordsings, they use bitrates all the way up to 320kbps for some passages, with most of the music recorded between 160 and 192kbps.

If you've previously ripped at 192kbps CBR, should you re-rip the old stuff? No, don't bother. For your future stuff, the only reason I can imagine not to use VBR is that the encoding time is significantly longer. CBR coding is fast. (Of course, if you drop the "-h" flag out of lame, it goes loads faster, but that's the "high quality" option...)