Most jet airliners (the Avro/BAE146 is the only currently flying exception I know of) need air pressure for starting the engines, usually provided by the APU. So using the APU is not a waste of fuel, just a mere necessity. Running it a few minutes before startup is often sensible in order to get the cabin temperature to a comfortable level and on occasion, the electricity provided by the APU may also be required or helpful.
And so, it depends. Extreme temperatures, expected large passenger loads (do not underestimate the heat output of a group of people!) or the lack of an external electric source will see the APU started rather early or run all through the turnaround while moderate temperatures together with low passenger figures will require the APU only for engine start and consequently started only a few minutes before startup. Formally, APU fuel burn is included in taxi fuel; expected heavy APU usage might cause a bit extra fuel to be taken though.
Last edited by Tu.114; 5th Oct 2013 at 17:27.