I used to prewash everything that came in the house, except fat quarters. Then I got too lazy to prewash the fat quarters (the washing/drying is easy, it is the folding that got me down!) The pre-wash advocates will tell you that the chemicals used in the fabrics can cause respiratory problems when you are working with the fabric, especially when ironing. I know there are some people that are very sensitive to this, so I respect their decisions.
I will prewash a batik or a hand-dyed, or a bright red, as I don't want them to violate their neighbor fabrics. I also pre-wash fabrics that have gotten terrible creased, like the wide backing fabrics.
As I am using some reds in my 2009 BOM, I will pre-wash the whole kit-and-kaboodle. I probably will be using a light spray starch or sizing when I am ironing them out, as I do like the crispness. And that is my story, and I'm sticking to it!
Maggie in E. Central Illinois