My thought is it doesn't really matter which side you put it on. You're stopping the flow of juice either way. To me the best way is the easiest/shortest wire from the battery. That being said in the case of my Cobra replica that I built I needed to have a keep alive wire for the engine computer and radio memory's so I had to put the switch in the pos side with a small (5?) amp fused circuit around the switch going to just those two items. If any kind of short were to happen in that the fuse would blow quickly.
