Varies from state to state. States quite clearly in our FOM that it can be used anytime after the refueler dis-connects, so that includes APU burn BEFORE dispatch.
411As diatribe-
Only a very foolish Commander would use his contingency fuel prior to departure.
IF he does so...back to the RHS, or better yet, shown the door.
With a swift kick in the backside.
Shows his companies culture is as geriatric as the aeroplanes he flies!!