A Muslin sample is a great way to fit a new pattern or an "experimental" pattern without ruining a nice piece of fabric. By sewing a sample made of muslin you are able to test the construction of a new pattern and it allows you to mark up the changes and adjustments during a fitting. It's really easy to refer back to your muslin when making the changes to your pattern.
I would suggest using a weight that is as close as possible to your actual fabric, as well as similar in weave or construction. Better to work with a less expensive fabric than to cut into that fabulous silk from France! (If only I had fabulous silk from France....).