When I makw a big quilt to cover the bed or myself, I don't iron it. It will probably be washed and change shapes soon anyway, plus I will never see if the sides are straight or the corners square. When I make a wallhanging that I will see and notice a slight curve on the sides or a less than perfect corner, I iron it flat before I trim to put the binding. After the binding is on I don't press any more. After a couple of shakes the binding fluffs again.
My two cents worth,
Maria