Hey Bethcote, your question is really common and is one I struggle with in terms of pressing to the side, there seems to be so many rules and variations and if/thens involved.
I solve it by pressing seams open. I'm like a hobbit and I like "one rule to rule them all". There are some very rare times when I do press to one side, for example in curves, always easier/better/smoother results for me when I press to the side. Some construction techniques (like one I'm using right now foundation piecing) require/result pressing to the side.
In pressing open particularly, but in all quilting you need to have a small enough stitch length in your seams. If your seams fall apart, or press apart or quilt apart that is due to stitch length, not to being open or shut.