I have been ironing seams open for a little while now, and I can tell you that doing it that way makes it so easy to stitch in the ditch when it comes to quilting. I reasoned that probably, the ironing to one side was because of hand sewn patches that weren't so strong. With modern machines, and the shorter stitches I tend to use, I reckon that my seams are not at all likely to part company in a hurry, especially as they are so difficult to remove if I make a mistake. There is also a lot less bulk in some places, compared to ironing to one side. The only draw back might be that you can't 'nestle' seams ironed in opposite directions when wanting to seam two sets of patches together. BUT even then, it's easy to pin the seams together when they are ironed open.