I used to use steam, but now I prefer to use just a very hot iron. It flattens my seams just fine. The problem with steam is that it is easier to distort the fabric, especially if you have a heavy hand and/or are ironing on the bias. Some people can iron with steam and get no distortion; others iron with steam and get a lot of distortion. Probably the easiest way to tell is to sew together some half-square triangles, then press half with steam and the other half without steam. If your steam ironed blocks are not square and your dry ironed blocks are, you know that steam is contributing to distortion.