Steam caused ripples in my pieced units. I also had no end of trouble with my various irons spitting and staining fabric (I can't lift heavier irons, so didn't have a great deal of choice in which ones I could buy). It was a relief when I stopped steaming and just started using a dry iron. I now use Best Press on preliminary seams when piecing. Sometimes I still get the rippling problem with the Best Press in cases where a fabric is sprayed multiple times because of later seams being sprayed near fabrics that were sprayed earlier.