I really think it is a matter of personal preference. Sometimes a seam doesn't want to lay the way you want it to and you have to go with the flow. One rule of thumb I do follow is never press a dark toward a very light, it will show thru. Sometimes here pressing open is best.
The reason traditional quilting was always pressed to one side was the strengthen the seam when hand stitching was the only way to make a quilt. With today's fine sewing machines and tinier stitches strength isn't so much a factor any more. However, that said, if you make a quilt that is going to be dragged around by a child outside and in maybe pressing to one side might be the only option.
Hope I was clear enough, I trend to ramble!!