Pressing seams open does give a flatter quilt top. I don't usually do it because I don't like depending on the row of stitching to hold the pieces together. I think it puts more stress on the thread pressing open but I do press open when a lot of seams meet in one place. In the old days, seams were always pressed to one side because the fabric weave was looser than modern fabric and the thread was poorer. In todays good fabric, thread and machine piecing and quilting it is less important, in my opinion. Do what works in your piecing situation.