Pressing to the side was necessary when seams were hand sewn, but it isn't necessary with machine sewn seams. A machine sewn seam pressed open will not be weaker than one pressed to the side.
However, there are other reasons to press to the side, such as nesting, and stitching in the ditch (there isn't a ditch to stitch in when a seam is pressed open). I prefer pressing to the side for a number of reasons, but sometimes it makes more sense with the pattern to press open (if, for example, you want both pieces of fabric to be at the same level, rather than having one raised above the other).