Phil mentioned seeing or hearing about this article recently, so he may know the source. I prefer the "position of greatest usefulness" philosophy. In general, I prefer to lead from the side or moving back and forth within the group. That being said, there are times when as the leader, you might need to take the lead at the front-to check out a potential hazard, etc... But often it's much easier to see and keep track of the group from the side or even the rear...