There is no electrical reason for mounting a OFF-ON switch in the POSITIVE WIRES or the NEGATIVE WIRES.
My installation has alot of connections on the POSITIVE SIDE so this was best solution for me. I could of just as easy mounted the 4-way switch in the NEGATIVE cables and accomplished the same thing.
In the automotive world disconnecting the NEGATIVE cable first was always a good idea as you usually had the battery is very tight location and using a wrench on the terminals could very easily touch frame ground in the process of removing them. If you touched the NEGATIVE TERMINAL to ground nothing would happen..
Putting the ON-OFF Switch in the negative side would almost require two ON-OFF switch for two batteries. You could connect the two NEGATIVE cables to a terminal block I guess and then install a ON-OFF switch between the terminal block and frame ground.
This problem also comes up when you want to add a SHUNT DEVICE for remote current meter. Best place for that would be in the NEGATIVE CABLE lead.