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!