Most mounting holes in motherboard have metallic ring round the hole
No washer needed above or below the board .
Optional star lock washer above board. (rarely seen now days)
Good operation requires the motherboard to be grounded to case via the mounting pillars
If there are holes without a metallic ring and there are copper traces on the motherboard near these holes a insulating washer on both sides can be used.
This is a very rare occurense.