I love to use muslin for backing, I just love the feel of it. I always get mine from Keepsake Quilting.

I have a quilt top finished but I didn't wash the fabric first - just by choice. I want to back it with muslin. Should I washed the muslin before quilting? Will it shrink more than the 100% cotton used for the top?

i just posted pic's of MSQ quilts. they both were made using bleached muslin scraps from cutting quilt backs. the $12.99 --116 in scraps. i bleached the scraps some more. and used it.

It depends which country you are in. Over here in the UK muslin is more like a very fine cheesecloth which would not be strong enough to use as backing.
Although UK ladies dumelm is not labelling muslin correctly. There muslin is more like 100% cotton fabric! cheap and strong enough for backing! too strong for backing of applique pieces before putting cotton backing on.