I almost always prewash. I don't do it until I've selected the fabric for a project. I like the ironing. It gives me an opportunity to review my selections and more times than not, I will swap out one of the fabrics. I have one exception to prewashing. If you are doing a Stack and Whack where you have to cut precisely in the same spot of the repeats, it is best not to wash. Also, if you are making a wall hanging, it is probably not necessary.