Sprint, in theory, I think a "passive" break-in should be better since the brushes will bed in without power arcing also creating extra wear or pitting on the comm. In practice, I usually break in motors at a voltage that minimizes the arcing anyway so the difference may be small indeed. Using the under-water method, break-in occurs very quickly and might even be better than the "dry passive" method since far fewer rotations are needed to bed in the brushes. But... it's all theory...
Lotus, it's tough to argue with the success you are having with your method, but I would think that the comm would be better served with a lower voltage. New motors at high voltage produce a lot of arcing which pits the comm and deposits brush material in the pits, creating a dirty surface that isn't easily cleaned by conventional methods. Lower voltage will also generate less heat, which is never good for a motor. Then again... as I said.... tough to argue with success.... but I had to say it.