I don't prewash at all. If you quilt adequately in relationship to the batting, the quilting actually helps the sandwich not distort. I don't mind a little shrinkage (whatever is normal in fabrics) because it just provides the soft, old-fashioned "crinkle" look I like.
In your situation, I might prewash the backing fabric, but this would depend on the fabric. Muslins and flannels can shrink a lot, so I would prewash. Most others are okay.
Washing the top is likely to make a mess. The only way I would ever wash a top is if I first basted it to a foundation fabric (to prevent fraying of all the seams), then hand washed and air-dried.
Whether pre-washing or not, it's a good idea to starch both the top and backing fabric before layering if you are planning to machine quilt. This helps prevents puckers. (Not necessary if you plan to tie the quilt or hand quilt.)