You don't 'have to' iron anything. It will depend on how experienced you are, how confident you are that you can hold down the seam tightly and are you doing it for a 20 minute video that can be redone or a gift for someone that you are concerned about the precision? I admit that I skip ironing every seam when I am making a quick item (placemats, mug rugs, etc), but, I am usually happier with my finished product when I take those few extra seconds to iron it.