I make my decision whether to prewash by what the finished piece will be used for. If it's for a baby quilt, then everything gets prewashed in hot water and dried in the dryer. I want to remove all chemicals first and foremost, and then I want to make sure that the quilt can be washed over and over without shrinking or colors bleeding. If the end use will be a wall hanging, then I'm okay with not prewashing because the likelihood that the hanging will get stained is pretty low. Placemats and table runners definitely get washed and dried, because they're going to be used.