If you're talking about a surface mounted box, with a surface 1/2" 'raised' cover, then some inspectors look at differently. Some will pass it if the cover is the same brand as the box (ie: Raco, Steel City).
Some look at as "the cover might separate from the box if the mounting screws loosen" (a definite possibility) and won't allow it.
To be on the safe side, run the ground wire from the receptacle to the box. Grounding pigtails are cheap insurance.
Well I guess I will probably add it, helps to hold the receptacle while I am wiring those boxes mounted on the ceiling!