I will soon be adding some outdoor wiring using weatherproof metal boxes and EMT conduit. I noticed when I was playing around with a multimeter that without a ground conductor between a receptacle and the box the receptacle ground terminal still read 0Ω to the box. I realized that the ground screw on the receptacle connects to the receptacle frame, which is fastened to the metal box, providing a ground pathway (as seen here).
If my EMT/metal boxes are grounded, do I need to pigtail ground conductors from the box to the receptacle, or can I use the mounting screws and frame to provide grounding?