You may also notice that more and more of us are pulling on stand with the No'2 eng shutdown. All designed to save fuel in this ever increasing climate.
We have (on the 73 that is) an indication telling us when we have ground power available. Once we have this we transfer the power from either the No'1 gen (if No'2 Eng shutdown) or the apu gen to the GPU, via a simple switch.
We may elect to keep the APU running for cooling, it all depends on the day!
My Airline requests minimal use of APU, mainly to save fuel and cycles - that is why we often do not shut down until GPU available, hope that answers your questions!