Robert, part of the reason for the cost of your copper roof is that it will be maintenance free for years. If you varnish it, sooner or later the varnish will wear away or flake off and you will be stuck with a part shiny, part green roof and you will then have to take all the varnish off. This will be a major job. Your current shiny copper will gradually weather to a pleasant green patina and will probably last for 50 years, depending on many factors. You might ask your installer if he thinks it is beneficial for you to coat it. Ask him the pluses and minuses and go from there.