I try to do back with cooridnating fabric or contrasting fabrics. For special ones I use upholstgery velvet, prewashed. it is much easier to use then dress making velvet. I have also used other upholstery fabric with great success. I was gifted 3 partial rolls of upholstery fabric. I do craft shows & clidents are always impressed how my tablerunners are finished on back. As others have said makes them reversable.
It just depends on what strikes me! I am currently finishing one that is one of Eleanor Burns Dresden plates on the front. I used elegant fabrics and it looks beautiful and formal. For the back, I had some fabric that has cow angels and Christmas gifts on it. I got 2 looks and used up some fabric that I have no idea what I was thinking. It turned out veru cute and can be used on the kitchen bar or the formal dining room table. Too cute!
I bought several yards of Kona black so if the table topper has black in it, that's what I use. I also feel like it won't show the dirt as quickly as lighter fabric.
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.
If I'm making a holiday table runner, I always use a complimentary fabric on the back. I have lots of fabric that I purchased when I first started quilting and would not purchase now, so those fabrics make great backings for table runners. Holiday fabric always goes on sale just before the holiday or right after. That's when I buy most of my holiday fabrics.
I go with the same/coordinating fabrics or if its a specific say Christmas on front/winter on back so you get holiday or season. I use my table runners. They get dirty if you flip them, another few days before I need to wash them. I always gift with coordinating fabric on the back. I love reversable so I gift reversable. That said muslim is fine if that is what you prefer to use on yours. There is a saying. there is a rear for every chair. What works for one wont necessarily work for another so go with what is right for you. It will be just fine.
Here's an idea. Since so many of us have colored dish sets today, use fabric the color of your dishes for the backing and bring your backing over to the top for the binding - if the color doesn't clash with the design on the top. Then make napkins out of that same fabric.