IMHO the whole car should be assembled, and stripped of all items not going to be painted body color, the only componets that should be jambed off the car, prior to painting, are the hood and trunklid, the entire body should be assembled and panel aligned, shoot the body/engine compartment/door jambs/trunk jamb at the same time, painted mirrors and such can be painted off the car at this time, depending on the color, your skill level, humidty, etc, etc, batch painting major panels off the car can result in shade/texture differences, best to paint the car as whole as you can, there are exceptions for each car/componets,...plus installing pre painted fenders/doors/etc always leads to that "uh-Oh!" boo boo