Dont the screws provide a conductive path ?

Using metal washers is fine (my mk2a has metal washers), provided that they're not much bigger than the screws. The danger is that the washers might be a little big and overlap with the PCB, potentially causing a short. This is very unlikely, though.

As you say, the screws provide a conductive path, so (right-size) metal washers aren't going to be an issue.

If you're worried, those little plastic washers used on PC motherboard standoffs would be ideal.