I have a bolt of navy blue 108" wide fabric that is my current "go-to" fabric for backing quilts, as long as it "goes" with the quilt well enough.
I also stash 108" wide muslin, for another "generic" option. I also have several pieces of wide fabric prints I've picked up, in a variety of colors.
Bed quilts I tend to make a more boring backing for, but lap/throw quilts I figure the back is probably seen nearly as often as the front so I usually snazz it up a little with some pieced bits using leftovers from the front if I'm piecing the back out of standard-width fabric; or use a nicer printed backing if I'm using wide fabric.