I'm so glad I asked this question. All of your responses vary so much. I used muslin in the very first quilt I made and it's still on my bed. It's at least 8 years old and gets washed regularly and is still holding up just great. No signs of wear at all. Hope it stays that way cause I expect to have it on my bed for many more years to come. It's not that pretty as it was my FIRST, but it is my favorite. Besides one other that hangs on the wall of my sewing room it's the only one I've ever kept.
I will continue to use muslin in my quilts, front and back. I happen to love the look and the feel of it. I do always wash it first to get all the shrinkage out of it. Thanks so much to all of you for all your varied responses. I look forward to reading more.