That's a good question. The bus is actually only in the neg side. The power source has a single point of generation and then splits out to each of the subsystems. The power source's ground side connects to the Neg bus.So how does the power source tie to the bus?
Because of this, I'd be concerned with simply placing the shunt between the power source Neg and the Bus, for fear of other subsystems bypassing the shunt (by nature of directly being connected to the bus elsewhere.)