.. If your computer case has metal feel which your mobo is supposed to sit on and screw into...then you MUST ensure that the cardboard /plastic washers are screwed between the mobo and the case's mobo feet.
The cardboard / plastic washers are supposed to eliminate risks of short circuiting your motherboard, through any metal feet on the chasis. If the feet are plastic, then you don't need the cardboard / plastic washers, but if they are metal...you don't want ure PC smokin' do ya !?