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.

38 Chevy 1/2 ton being resto-moded
65 AC Cobra replica
USAF vet