You need to ground any metal parts. In this instance, if you had a portion of the energized wire touching the metal box as a result of the wires insulation being stripped, you in effect have a box if ungrounded that is energized and could be lethal. For commercial, we still only use metal. The conduit is our ground. For residential, I try to use only plastic or non conductive boxes. While it is simple to ground metal boxes, it still takes time. I typically pigtail the ground wire in the metal box before I ever install it. You will notice that metal boxes already have the ground screw hole drilled and tapped. Add about 8" of ground wire to the screw and your set.
I still only use dielectric grease for aluminum. It is the aluninum oxide that presents the problem for elecctrical flow. If your metal and copper are getting heavily oxidized, you don't have a oxidation problem, you have a bad mositure problem that needs to addressed. I have had an electrical inspector make me remove dielectric grease on metal to copper and CU to CU connections. It was on some large hydraulic crimp connections on 750MCM CU. I was not pleased and now only use it when it is called for.
