Prewashing of quilting fabric is a subject for debate among quilters. Those in favor of prewashing do so to avoid the possibility of the excess fabric dyes running and 'bleeding' onto a lighter piece of fabric and ruining many hours of hard work. Another reason to prewash all your fabric is to be sure that any shrinkage takes place before your blocks are cut out and sewn together.
Fans of not prewashing cite the crispness of the fabric as the top reason not to wash before piecing. If all your fabrics are washed before piecing, they will tend to shrink a little and give an old-fashioned 'puckered' appearance.