Whenever board to aircraft. Just after fuel is loaded and truck disconnected, of course. Ground power which is 28 volts usually don't provide air conditioning. APU is more reliable for AC most of the time. Considering it a waste of fuel is not reasonable, as Performance Initialization calculates all the APU fuel burns for the reserve fuel, but of course you can change it via cross-feed from a tank which required. After landing, APU is started in a time between exiting the runway and shutting the engines down, mostly on taxiway.