Not so fast oh wisher for the realism
The flaps and gear doors bled down slowly after power off. Keep in mind before a mission the Crew Chief and his assistant would be out prepping the 51 for the next mission, including running it up and topping off the tanks among other things. The pilot would come out to a 51 ready to roll, including the flaps and gear doors back up because of the run up
Any mission the cartoon 20th flies, we'd assume your cartoon crew chiefs would have already done their job before you guys take off. It would look 'wrong' for you to have your 51s unprepared with the flaps drooping and gear doors down. Post mission, after a few hours? Sure
A line up of Red Tail P51Cs ready to start up. Note the flaps and gear doors up, props not moving. Crew Chief on the wing, pilot in the cockpit waiting for the word.
20th Fighter Group P51D also topped off and waiting for the pilot to arrive for the mission
4th Fighter Group P51D, also topped off, run up and waiting.