Muslin is perfectly good for backing. Who would turn it over and be critical, and if there really was such a person, why would you care? Another option, which I have used because it saves space in the linen closet, is making another pieced table runner for the reverse or using a seasonal novelty fabric. I have a fall one that reverses to Halloween fabric and different Christmas ones back-to-back, and one in spring colors reversing to an Easter design.