Q:

Switch placement on electronic circuits on the positive wire

Why are main on/off switches in electronic circuits/appliances (any small, low power electronics) always placed on the positive wire (in case of DC circuits) between the power supply (batteries, adapters, etc.) and the circuit regardless of the low operating voltage? 
Does it affect the efficiency and makes any difference in power consumption when it's turned off?
Wouldn't placing the switch on the negative wire between the power supply and the circuit just work the same?

A:

Why do main on/off switches in electronic circuits/appliances (any small, low power electronics) are always placed on the positive wire (in case of DC circuits) between the power supply (batteries, adapters, etc.) and the circuit regardless of the low operating voltage? 

Mostly, convention. You connect ground to the negative supply, because that's what all the ICs expect. Then, you decide that it's easier to switch the pole that is not ground.

Isn't placing the switch on the negative wire between the power supply and the circuit just works the same?

usually, yes.

