When I first learned about quilting, the teacher said to prewash the fabric, and that seemed normal to me. I later learned that it is a preference, some people do and others don't. I still pre-wash everything,not only for shrinkage and getting excess dye out, but for sanitary reasons. After all, I do wash linens, towels and clothes as soon as I bring them into the house, same goes for my fabric. It's a lot for work (the pressing mainly), but it is worth for me, I am more comfortable working with the fabric.
With that said, I avoid buying kits because I don't want to take that chance on either not washing or not having enough after I wash.
Just my two cents :) And BTW, I am a new quilter, so no long experience here.