It depends on the use of the quilt for me. I use muslin for backs of wallhangings as they are not likely to be seen, and some colors of fabrics when in humid environments can leave a color tint to the wall if left there ong enough. (Yes, I have found some banners at churches that have done that, and especially when men think they can nail the banners directly on the wall!)
If I am giving a baby quilt, I usually use flannel in the same color family or theme as the front. i use backing fabrics of tone on tone prints alot for bigger quilts.
I think the back needs to be pretty also, but that can be scrappy or not.