I almost always prewash, a carry over from days of sewing garments. My concern of not prewashing is unequal shrinkage. If you are using a collection of fabric, this may not be an issue, but if you are using fabrics from different groups and different manufacturers, there is a major difference in the amount of shrinkage. I have come across an issue with a pattern, that was obviously designed for not preshrinking. The pattern was for the collection I was using and required 3 WOF in the top. After preshrinking, my WOF was nearly 2" shorter than the quilt top called for. I was able to piece the strips fairly easily, but it sure convinced me of the need to prewash my fabric.