I generally don't prewash but I do starch and steam-iron. A block I made yesterday from yardage and pre-cuts was so off that I took it apart and re-measured my pieces. All of them had shrunk from the pressing the seams just enough to make the block wonky. So I put the yardage (Timeless Treasures) in hot water and hot dryer - it shrunk 3.25 inches! cross-wise. I put the pre-cuts (Moda) in the sink - no agitation, just to wet them, and they raveled so much as to be useless. So lesson learned, pre-wash yardage, do not soak pre-cuts.