advantage of filling a block

Ran one on the street years ago with no issues. Granted, it was a mild build, someone gave me the block so I just used it. As I understand it if you only fill to the freeze plugs it "should" not effect the cooling, unless you cooling system is borderline to begin with. Reason being the majority of the heat is towards the top of the cylinders courtesy of the combustion precess. This is how an old timer explained it to me.