Unlike Swiss cheese or mozzarella, cheddar doesn't become stringy very easily. But if you'd like to make sure that your cheese sauce won't become stringy, stir it over low heat just until the cheese is melted and no longer than that. You could also squeeze a little fresh lemon juice into the sauce. I believe that the Dijon mustard, an acidic ingredient, also helps prevent stringiness. So use it! It adds flavor AND helps with the texture.