Ok, this is kind of a weird issue, but I wasn't paying too much attention to the command line options I set for LAME in grip, and ended up ripping and encoding a significant number of CDs with a variable bit rate (at V0) AND a minimum bitrate of 320k. Kind of pointless.
I then tried ripping two files with CBR and a minimum bitrate of 320k just to see the difference, and the file it created was exactly the same size as the VBRed one. So then I use the cmp command to see what the difference was and I got:
[the first two files] differ: byte 2080, line 2
[the second two files] differ: byte 2289, line 2
I know very little about the actual mp3 format; does this byte designate whether it's CBR or VBR?
I know it seems strange to obsess over a single byte, but I wanted archive quality for these songs, so I was hoping for perfect encoding, and like I said I know very little about the mp3 format so I'm not sure if there will be any difference in how mp3 players handle the files. I tried playing both in VLC and the VBRed one seems to use slightly more processor time, but top is sufficiently imprecise that I couldn't be sure.