Valve cover gaskets are just a general pain to begin with. Don't use them at all. Use silicone instead. It's a much better solution. It isn't subject to uneven compression and cracking like many are.
Cork gaskets are about the worst solution. They don't compress evenly and are not reusable anyway. Just glue the covers down with silicone. You can easily crack the valve cover as a result.
Your shop didn't suggest this to you? They should have unless they did and were over ruled by an MBA?