Even if you use metal boxes, don't forget the recepacle isn't grounded to the box if the box itself isn't grounded. Besides the equipment grounding conductor being bonded/attached to the green grounding screw on the receptacle, you would need to run a pigtail over to an approved green ground screw on the metal box or an approved clip so the equipment grounding conductor going to the main panel is bonded to the box. If the box isn't grounded then there is nothing for Chris to feel secure about and would be false security. Since the equipment grounding wire is bonded directly to the green receptacle screw, metal or plastic boxes doesn't matter. Metal is needed if you are using metal conduit but that is not the case here.