No (mostly). The mounting holes on the PMDX-424 are not connected to anything on the circuit board, so grounding them would not accomplish anything anyway. Depending on your configuration and wiring, you *may* end up connecting the ground for the isolated inputs (J16 and J17) to the machine ground, which in turn may be connected to the earth ground. And that is OK because those inputs (and their ground) are isolated from the rest of the PMDX-424 (i.e. processor and USB interface). But under normal circumstances, none of the "common" or "ground" terminals on any of the other connectors (J7 to J13) should be connected to earth ground.
