If you haven't washed the top, I wouldn't wash the bottom. I'm a quilter who never pre-washes anything and I've never had any problems and I mix a lot of fabrics in my scrappy quilts. Both the top and bottom may shrink a little but they will be stitched together and can shrink together if they must. I don't see any value in just washing the bottom. You'll get lots of opinions on this one and I don't think that there is one correct answer.