With the advent of Lame 3.90 a lot of the guesswork has been incorporated into the --alt-preset settings. I used to be die hard 192 CBR, but decided to switch to VBR for the reasons expressed under this thread. If space is an issue then LAME supports ABR, which is like VBR, but the file averages out to be the equivalent size of a CBR (e.g. ABR 192 will be the same size as CBR 192).

At the end of the day it depends on how much space you have on the unit and how many songs you have. On my 10Gb I have 1400 songs which are encoded using '--alt-preset standard' (which is VBR at about the 220kbps, enough to give it WAV->MP3 compression ratio of 7.3->1). If space wasn't an issue then I would have encoded using either the 'extreme' (@VBR256) setting or even 'insane' (CBR320).

If you were being scientific about it, you should encode a couple of tracks at various levels and listen to it using good quality headphones. See which one sounds perfect to you (e.g. 192kbps) and choose to encode at 1 level up from that (in this example 224kbps). As long as this is below 320kbps, then I would use VBR, then ABR and then CBR.