It may not be necessary to block before sewing the pieces together, it can make it easier to seam if the edges are curling. But it depends on the fiber too - acrylic won't 'block' by pinning and wetting, you'd need to steam them and be careful that they don't get too hot or the shape changes. You can also just put them together then wash and dry afterward. For acrylic or cotton that's in the washer and dryer, for other fibers wash by hand and lay flat to dry - you don't have to pin it into shape.
sue- knitting heretic