I've been taught to only use steam when I'm first ironing the fabric to prepare it for cutting. The wetting & de-wetting process does typically cause the cotton fibers to shift. Of course, there are many factors that will impact how much that will alter/distort your edges/seams. If you are using steam, it does help to allow the piece to completely cool & dry in place before you pick it up after pressing. As others have stated, try to go straight down (press), rather than moving the iron around. If you heavily starch the fabric at the beginning of your project, that will help to limit how much the fibers shift. And, if your pieces are cut on the straight of grain, rather than on the bias, the distortion will be much more limited -- especially if you use good quality fabrics.
I will say that I seem to get better results with a mini iron for pressing seams. It does not use any steam & is lightweight, thereby minimizing the distortion that occurs when I move it slightly to make sure the top of the seam is pressed open.
That said, the main reason I switched to a dry iron in the first place was to avoid steam burns as I was trying to hold down my fabric. If you're happy with the results of steaming your seams, there's no reason to switch. Clearly, you have a technique that is working for you.
