As to a gounding plate. I am very fimilar with electircal engineering terms since I do have a BS in EE. But, If you have your bord set off from the plate (and for a quiet pc I have seen use of plastic off sets) and the spots where the screws are attched to the off sets are issolated from any part of the circuit board how would that ground your board?
There's supposed to be at least one metal standoff, which grounds the board through the metal bolt. However, there are ground wires in the PSU connector to the motherboard which provides all the grounding you really need.
If the board it's self conducted then you would always have a short wouldn't you? Second, what is gounding the plate. In general you need to have a natural ground to be trully grounded.
If a piece of metal contacts the bottom of the board, then you will short circuit the motherboard and it won't function. If you're lucky, it won't be damaged in the process.
The metal plate behind a motherboard is typically grounded through the rest of the computer case, ultimately in contact with the metal case of the PSU. Inside the PSU, there's something connecting its metal case with the internal ground. Once upon a time, this ground may have been important. Nowadays, it isn't.