I'll try to help on this and describe what I do and why. Others can comment on whether they like this procedure I use.
1. I rip from a different program (CDex, not MC12). Only because I have not experimented yet with this aspect of MC12.
2. I then use MP3Gain to change the ouput level of each file. This works well with MC12 and other players. However, I no longer stop there.
3. I then analyze these files (already adjusted by Mp3Gain) in MC12 and turn on the volume leveler. This allows me to use the leveling features of MC and use the leveling features of my IPod when I export or burn to disc.
I have not notice a discernable difference between CDs burned in MC12 with the files actually adjusted by MP3Gain and CDs burned usng the leveling feature on MC12 on these same files. Technically, there shouldn't be a difference because it is my understanding they use the same methodology. When I view the audio analysis results of an MP3Gain modified file that is then leveled through MC12, the results indicate that (with my settings), MC12 reduces the volume about a 3 db with little variation. I conclude that my settings between MP3Gain and MC12 are about 3 db off but that the music was leveled both before and after (there is actually a variance of about 0.5db from song to song - a non-discernable amount.)
So... In conclusion, leveling in MP3Gain and modifying the files (and creating a CD with volume leveling turned off) seems to yield about the same result as analyzing those files (in MC12) and then turning volume leveling on in MC12.