Pressing to one side was done originally due to it prevents the batting coming out of the seam. I think that was way back before the batting making technique was perfected. I press them to one side and if you are stitching in the ditch you'll notice it's easier than if they are open. I think for the most part now days, it's a preferrence; like just about everything in sewing and quilt making. I'm not sure if it makes a difference if you ever plan on entering them into a quilt competetion.