I don't usually prewash. But with a collection of CT solids, when I spritzed with water and pressed with a hot iron, the amount of shrinkage was pretty shocking. I measured the shrinkage to be 3 inches in one direction, 2 in the other on a fat quarter.
If you are mixing with other fabric lines, planning to make a quilt with heavy use, or are a real stickler for precision piecing, I think, based on my experience, it's safer to prewash.