I've heard that people were taught to press their seams to the side back when they were hand sewing and the threads used weren't as strong as we have today. They said that the folded over seam was stronger and had less stress on it. Since we almost always piece by machine (I do, anyway) it doesn't really matter which way we press. The quilting we use also adds extra strength and there is very little stress on the actual seam. I say, do it whichever way you feel more comfortable with.