You will find people pretty evenly divided. I don't prewash. Not even batiks, and I've never had a problem. So, it is entirely up to you. If you want to wash, and dry, and iron, and starch and iron some more before you cut and sew, go for it. I don't have the time, energy or inclination. Unless you have terrible allergies to new fabric, there is no need to do so. You'll have people tell you horror stories of "my fabric ran and ruined my whole quilt", but that can be avoided by washing in cool or lukewarm water, making sure the washing maching fill is set on super, and throwing a couple of color-catchers in.