for my special quilts, I am attentive to the back. however, I have been known to do the random fabric piecing you described. I have done that on my less-than-special quilts and i get pleasure at using up bits of stash that I wouldn't otherwise use. I do try to to stay within a reasonable color theme though.
I made my mom a quilt from a moda kit that she picked out. there were enough fabric bits leftover to put together into a back. I cut them into similar sized bricks and pieced it together. when she opened the package, she thought the back of the quilt was the front! guess it didn't turn out too gaudy!