I always iron mine after I wash it. I always try to remove the deep set fold lines. I really like Dunsters' way of doing it, It would save a lot of time. I am just afraid of wetting it too much.
I don't iron customers' backing unless it is very bad, I let them know it is their responsibility to bring the backing ready for quilting. I do have a friend that does not do a good job of pressing/ironing and I will redo some of hers..(she keeps me busy )