I prefer spray sizing, because it does not draw any insects. In either case, the idea is to have the fabric relatively stable when you are cutting and handling it. The more intricate the pattern, the more this matters. The spray also tends to get rid of any wrinlkes, as you iron it. I bet the next question is why wash and iron. The washing is to preshrink and get rid of exess dye. In some cases, it's to identify fabrics that would run in splotches over your quilt, making a real mess after all of your work. If you don't iron, your measurements can be WAY off because the fabric is not really flat. I'll be honest. I hate ironing, but I am really convinced that it matters.