When you're blocking a garment that has ribbing (1x1 Rib, 2x2 Rib, etc.), lay the garment flat and re-shape to the proper measurements, but leave the ribbing alone. If you're pinning the pieces or garment in place, pin just above the cuff or waistband ribbing, or just below the neckband ribbing. This lets the ribbing draw in and keep its stretchy quality. If you stretch out ribbing, pin it, and let it dry, it will retain the stretched-out look. Sometimes you might want this, so by all means, go for it! But if you want your ribbing to be stretchy, just let it dry while the ribs are gathered together.