I prewash all my fabrics not only for shrinkage but to remove the chemicals in the fabrics, most importantly formaldehyde. Also, fabrics from different manufacturers will shrink differently. Some shrink lengthwise more than crosswise and vice versa. Prewashing fabric makes it much easier to reset the straight of grain for the fabric as well. If I use a cotton, cotton blend, bamboo, wool or any other natural batting I will preshrink the batt. I will spritz it very well with a water bottle and then put into the dryer with a wet towel and dry on my highest setting. Batting will shrink in all kinds of directions, and I don't want my finished quilt to look strange after it's first wash.
I am a very strong supporter for prewashing fabrics. This will not happen to most people but this is something I think we should all be much aware. Most woven fabrics have a finish that is applied before it is rolled onto a bolt. This is to prevent the fabric from rubbing holes into itself while on the bolt and being handled. It is also to prevent dirt and oils from hands discoloring the fabric in the stores. That finishing solution has a lot of formaldehyde in it. The formaldehyde prevents the fabric from attracting bugs, and kills any larvae that may still be in the threads. Formaldehyde can cause reactions in people, i.e. light coughing, running nose, stuffy nose, itchy eyes and in rare cases skin rashes. There are other more severe reactions but those are pretty rare. In my case, it is my eyes, actually more my eye lids. The scratching has caused all my eyelashes to come out, (i have actually scratched them out) and my eye lids are very swollen. I have to wash my eyes with baby shampoo 3 times a day (no makeup for me) then apply one drop of medicine in each eye, and apply an ointment to my lids.
Sorry to be so windy, but I really want others to know.