I've made hundreds of table runners as sell at craft fairs and to give as gifts - I like using either a coordinated fabric on the back, or if it's a Christmas or Fall/Thanksgiving table runner, I like to make the back another "holiday or season" so that the table runner can be used for more than just one holiday,giving it extra time to be used on the table, or if they just want a change. On wall hangings, muslin is certainly appropriate, as nobody sees the back of the wall hanging, so use muslin then and save the other fabric for items than can be seen on both sides.