I don't know for certain the protocol, however I can offer a few reasons why I am pretty sure aircraft were not left overnight loaded:
-- Fabric ammunition belts were susceptible to humidity, distorted belts led to jams, airmen did not like jammed guns. At all.
-- Fuel and coolant (and I believe oil) were completely drained from the aircraft every night. It stands to reason that the extra work of unloading the guns would not be unreasonable to expect.
-- Pilots were known to personally inspect their ammunition round by round before it was loaded into the aircraft. Again, this speaks to how seriously the ammunition was handled.
EDIT:
I misread your question slightly. My hunch is that the aircraft were prepared for combat on a daily basis when there was a chance of flying, which would mean filling all fluids and loading the ammunition. At that point I assume the aircraft would be left in a readiness state until a mission was flown or the staffel stood down at night.