I always wash all my fabric as soon as I get home from my
LQS. Washing removes excess dye and chemicals on the fabric.
Fabrics shrink when washed, but they all shrink at a different rate. If fabric isn't prewashed the fabrics may shrink differently when the quilt is washed the first time. Some fabrics have a lot of chemicals on them and is necessary to prewash to prevent illnesses like chemical sensitivity and formaldehyde allergies.