Are you thinking of using muslin to save money? Or because you think the back of quilt should be plain? A king size quilt is a great accomplishment, and it should be celebrated with a fun backing. That way, it's almost as if you have two quilts in one. There is nothing that says the back should all be the same, plain fabric - not even the same printed fabric. I started piecing the backs of my quilts - mostly just in large fat quarter sized chunks that coordinate with the front - but they don't have to. Yes, it's a bit more work, but when you pull that quilt up and can see the fun underside, you'll be glad you did it.