I have always used the metal screws and no washers. Not all cases kits include washers, and they're not for insulation. In fact the motherboard ground planes (which is what the screws mate down onto) are supposed to be connected to the case in mt view.
The only thing that I beleive the washers are for is to prevent you tightening the screws too much and damaging the motherboard.
My recommendation (that I've done on hundreds of PCs that I've built) is not to use the washers - and snugly fit the screws, but not overtighten them.
However, I'm inclined to think it makes little dofference either way.