To break off any circuit, the common way is to disconnect the negative and or ground terminal. I rarely see people disconnecting the positive and if so, they're not well educated in electrical engineering most of the times. 

Well, I've no electrical degree either and to me, breaking the circuit off at the negative side seems counter intuitive. 

In my intuition, breaking the input makes more sense than breaking the output, due this is the side where the energy is coming from. 

Why to disconnect the negative/ground terminal, not the positive?