I was taught to press seams to one side - toward the darker fabric if possible, and I've always done it that way. I do believe the seam is stronger. And, if you quilt by stitching in the ditch, and you've pressed the seams open, where is the ditch?? If you just quilt up the seamline, you are basically quilting the thread, not the fabric. There are ways to press which avoid the pile-up of lots of seams.