Originally posted by: CheesePoofs
You don't need to use washers at all AFAIK. One of the purpose of the spacers/screws is to ground the motherboard, so the metal on metal (screw on motherboard) can be a good thing.
PC motherboards do not ground through any of the mounting points. You could use all plastic stand-offs and it would have no effect on electrical characteristics of the system. That's what the wires in your ATX power connector and the mainboard's ground pathways are for.
The washers are to help prevent the screws from roughing up the surface of your motherboard or cracking it when tightening, and to inhibit corrosion between the differing (and potentially reactive) metallic properties of the screw and solder ring.
If using metal screws and stand-offs, you're getting metal-to-metal contact with or without a washer.