The Maxine Rosenthal book does say to use it unwashed.
I had washed my fabric long before I read the book!!!! When the LQS told me unwashed for the course, I went *gasp*.
The teacher said, oh not to worry .... just starch it up good ... that it was the slippery-ness we needed to allow the layers to shift easily when putting them together.
And bonus ... with all the starch there was barely any fraying throughout the whole process. When I took it to hte LAQ, she commented on how clean and tidy it was. Kind of sold me on the reasons why so many starch/size as they go.
Though I hate the starch ..... for the white flakes that come as you iron it. Would never spray it on the right side of the fabric.
And for those who are wondering about this OBW ... here's the link ...
http://www.quiltingboard.com/t-141548-1.htm