Valve cover Question

if you don't do it, you will not be able to get them to seal

put the valve covers on without the gaskets and you will see that the "lip" interferes with the runners
if you do not notch that you will not be able to get a good squeeze on the gasket in those spots and they will leak there

if you notch the lip so that the covers sit on the heads (without the gaskets in place) without interfering with the runners you should be golden