The topic of using muslin for quilt backings has come up a few times in the past week. In my opinion, a 100% cotton fabric which is the same quality as the fabrics used for the quilt top should be used for the back of the quilt. I have nothing against muslin, it is useful in many situations but it typically has a lower thread count than quilting fabrics so, although it is an inexpensive fabric, it is not an ideal choice for quilt backing.
If you use a low thread count fabric for the back of your quilt you could find that little tufts of batting will poke out of the back of the quilt.
Quality fabrics with a good thread count (the number of threads per inch in the fabric) are known to last longer than fabrics with a lower thread count. Muslin can wear out sooner than other fabrics which can cause weak spots or holes in your quilt. If you use muslin for the backing or binding of your quilt, these parts of the quilt will fall apart long before the rest of the quilt is worn out!!!
I don't know about you but if I'm going to put my time, money and effort into creating a beautiful quilt, whether it is an heirloom quilt or a drag-about, I want it to last as long as possible. It only costs a little bit more to use a cotton quilting fabric for your backing and binding rather than using muslin and you will save money in the long run because you will not need to make expensive repairs to the quilt.
When I first got my longarm machine I made some practice pieces using a cotton solid for the top and muslin for the back. I practiced quilting pantograph patterns and overall designs on these pieces. My family has been using these practice pieces ever since but the muslin backing is actually falling apart while the tops of the "quilts" are still in good shape even though they have been in and out of the washer and dryer many times. I would be very sad if one of my pieced quilts fell apart like that!!!