I would prewash for a couple of reasons. As mentioned you want to avoid bleeding...I always add a Tide color catcher sheet to the wash just to be on the safe side when washing fabrics that are not the same color. Second, there are less problems with fabric distortion around quilted designs and in the overall quilt. I always use 100% cotton fabric in all of my quilts because of the softness, and for a baby blanket it would definitely be the fabric I would use.