I was told a long time ago that the rule is don't put sealer between a gasket and a machined flat surface. If one or both sides of the gasket goes against a part that isn't machined, put sealer on those sides. With silicone sealer, most people put it on both sides of a gasket anyway where it's appropriate, like on a water pump or thermostat housing. It's easy to remove and it's extra insurance.