Good question. The logic I would use behing a cutoff would be on the + side. That creates a total diablement of the electrical system anywhere after the cutoff. If you used the - side, areas of the electrical system could energize if they were supplied a ground. (Keep in mind that this is a what if condition.)
One advanage I THINK would be behind the neg mount cutoff is a reistance issue. Again, I THINK that it would create more resistance breaking the + side, but you would need to confirm it with someone that knows a little more about electritity.
