In my 45 + years of painting experiance, I've done it both ways, and I'll tell ya this -
Paint parts as they hang on the car !!!!!!!!
Solid or metallic/pearl coat.
Think about this- Ya shoot the body, then ya shoot the door laying flat., go to assemble and the flow on the door will not match the body side.
You'll have urethane wave in the body more so than the door.
But what do I know, I've probably painted more cars than 50 of ya put together !!!
Hang the parts Roger