Is it a requirement of the code to have rubber sealing rings on the threaded portion of a EMT compression fitting? I've noticed recently that they are available in at least the 1/2 and 3/4 sizes. All the years that i've used EMT comp connectors and couplings I've never seen the rubber washer right on the fitting. Always bought sealing washers when I would be going into a panel or enclosure. Is this just an added feature by maybe a particular mfg, or is it a requirement. Thanks.