I have 3 CO2 regulators. One uses that washer and two do not. Using the washer in a regulator that doesn't need one will cause a leak just as bad as not using a washer when you should. I have made both mistakes.
You can tell the difference by looking inside the regulator fitting. Some will have an O-ring pressed into a groove on the flat face of the connector. Do not use a washer with this one.
Wayne