From what I've learned from this board, it is more about quilter preference. Pressing seams open allows for less bulk than to one side. Apparently, although I'm not sure if this is really the case, if you press to one side, there is less stress on the seam. I prefer to press seams open because it is easier to match up seams, but when making a log cabin, it was easier to press them to one side. I would suggest you do what works best for you and makes the quilt easier to piece together.