I wouldnt have any issues with using Muslin on the back of a quilt.
After all, it was done that way for a hundred years or more and those quilts have held up nicely..
It is more cost effective..
And, I hadnt thought about the fading issue that babeegirl brought up, but that is a good point too..
Depending on the look you are going for, you might want to prewash it as some muslin will shrink more that other fabric, depending on the thread count and whether it is prewashed or not..some muslin says prewashed...
Good luck!