I wouldn't prewash. As I've mentioned before, fabrics on their own can shrink a *lot* when washed. However, once in a quilt that has moderate quilting, the quilting binds the fabrics to the batting and the batting becomes the controlling factor in shrinkage. In other words, after moderate quilting, the fabric shrinkage is limited by how much the batting shrinks.
Shrinkage can be a problem if the quilt is tied instead of quilted, or if the quilting lines are far apart (for example, the 10" maximum allowed by Warm and Natural). In these cases there is not enough quilting for the batting to control fabric shrinkage. With a normal amount of quilting, though, fabric shrinkage becomes a non-issue.