motherboard and case problem

I have a computer case that has holes for spacers/mobo screws but they are projected upward (all the holes for mounting a ATX mobo are bubbled outward). if i use spacers the motherboard will sit too (x2) high up.
if i dont the motherboard will make contact with the metal of the case. what should i do? should i just put those little red rings (what are they called?) on the screw holes under the motherboard?