I never use to prewash. I suffered one quilt that shrank more than I calculated it would. But, got through it ok. Then, last year had some allergy testing done for problems that I had been having for years. Come to find out the only chemical that I am allergic to is the formaldehyde that is used on fabrics. Now I am a pre-washer. I actually wash all my fabric twice in hot water before it is allowed into my sewing room. I also buy fabric by the bolts a lot of times. Those bolts are not allowed into the sewing room until I cut off a piece, then wash it.
Since I have been doing this, I have a lot less problems, but not being around unwashed fabric has made my reactions worse now. When I go into any of my LQS, I usually last about 15 minutes before my throat goes scratchy, then I get hoarse and evetually lose my voice if I stick around too long!

I usually pre-wash all my fabrics but sometimes I have been known to be making a project and decide to use an unwashed fabric instead of one I had planned to use. In that case I usually give it a good steaming with the iron and pretend it is shrunk enough to go in with my pre-washed fabrics. It is a dark color going in with lighter ones then I might put it in the sink with some hot water to see if it is going to bleed first.

I don't prewash!

I prewash. Most of it comes from the thrift stores.

I pre-wash - no telling what chemicals will rub off on my skin when I work with the fabric

Oddly enough, all the Jinny Beyer fabric I have bought had a sheet of info saying to wash before using because of possible dye bleeding.

I am taking quilting lessons now. My fabric was raveling all over the place. I asked the teacher and she said it was because I didn't wash it first. She always washes first. I never heard anyone else on the board complain about this. The fabric was bought from the quilt shop where I'm taking lessons. Is that a logical reason? Thanks.

WASH! Made the mistake only once - and never again (and yes, I used a color catcher).

I prewash my fabric. :-)

I never pre wash. It started because I want to use my new fabric right now, I can't wait on the washing machine. I usually use a color catcher the first time I wash. They are available in the laundry detergent aisle in the grocery store. In my store they are made by Shout and are near the stain removers.