The following link is for a paper that was published in the Journal of Neurotherapy in 2007. It describes the benefits, as well as limitations, of monopolar and bipolar training, and makes the case for consistent use of monopolar recordings for assessment as well as training. It was written by Les Fehmi and Tom Collura. Even those who emphasize bipolar training methods are now coming around to using monopolar leads, and using sum and difference training as described here.
(This paper was invited by Jay Gunkelman, and was reviewed by Dr. David Kaiser)