In the past I didn't prewash my fabrics, unless they were dark colors or bleeders.
However, I changed my mind a few years ago when I started coughing and tearing up when I worked with the fabric. I worked in a LQS for a few months and that's when it started happening. A lot of the fabric has Formaldehyde and other chemicals in it, which is not good to breathe. Many people have alergies to it and then don't even know it.
Since then I prewash all of my fabrics, adding Shout's Color Catcher to the wash to get any extra dyes that want to bleed. I fold it up as soon as it comes from the dryer to keep the wrinkles out. I don't iron the fabric until I actually need it and only iron the amount I really need. For example: I have a yard of fabric, but I only need 1/4 yard of fabric, I cut that amount off and iron that amount. While ironing I use a heavy duty starch on the fabric, which gives it the feel of new fabric, making it easier to cut and sew on.
In short, one vote for prewashing.
mac