Grounding racks was always a quandry for me. Many of them come with a threaded hole and a ground mark next to it and the manufacturer mentions this is the ground lug. Many rack mount PDUs can be electrically isolated from the rack, along with all the items in the rack that plug in because they have painted ears and if you use non-marring mounting screws with nylon washers on them, that screw is effectively isolated from the rack mounted item.
Some network and telecom vendors want their equipment grounded and even have separate lugs for that. It could be that is in case they are powered by 48 VDC because otherwise the equipment ground in the power cord should ground the equipment. Shielded cables need to be grounded too so that ups the requirement if you have that and a patch panel for them.
So what I typically did was use Panduit rack bonding screws to mount the PDU to the rack. These have thread forming threads and little teeth under the screw head to bite into the rack mounted item. I would also mount a ground bar in some racks with those same bonding screws. If the PDU had a ground stud, even better as I'd run a jumper from there to the ground bar. Rarely, I would run a separate #6 from a ground bar on the wall to the ground hole on the rack. This seems redundant and most of our racks were at least somewhat moveable so a #6 green wire going into the floor or secured above to a cable tray was like a leash and not desired.
No one from state or federal OSHA inspector ever seemed to care about rack grounding, so this was just an over and above that I did. Probably because early in my career I had a rack with no wheels that I pushed and it went over a power cord and cut it. I saw sparks, but that popped the breaker and the rack didn't get energized as far as I could tell. But I tried to not let that happen again.
