Ditto what Julie says. I prewash all of my fabric. First is to remove all the chemicals in the fabric, excess dye, formaldehyde, insecticide and the like. Second, fabrics do shrink at different rates some 1%, some 3% and some at much at 10%. Prewashing prevents the quilt from becoming warped as it is used.
Some do not prewash because the "crisp" fabric is easier to cut. I have found that if I starch and press the fabric before cutting it cuts much smoother than non washed. Also, it seems like my rotary blades last much longer this way.
If I am using a cotton batting or cotton blend batting, I mist the batting with water and throw it in the dryer with a wet towel. This removes all creases in the batt and preshrinks the batt as well.