The washers provide for expansion (and the foil exhaust manifold gaskets go on only one way to enable the manifold to "slip" a little during heating and cooling expansion/contraction). The end studs/bolts do require a sealant because of the water jacket.
When I first installed a rebuilt small block in my '65 Barracuda, I could hear a "sizzling" noise when I shut off the engine. I later found that the rear passenger side exhaust manifold stud was not sealed and the noise was a mini-leak (maybe steam) coming from that fastener that required some or more sealant.
I believe the factory did not use the special washers on the two center manifold bolts (only on the other four outer ones), although I find "kits" sold online provide the washers for all the fasteners.
