It's mostly just a matter of usage, though in my personal (unwritten) style guide, 'law' would be reserved for constraints that, if not strictly true all the ti

It's mostly just a matter of usage, though in my personal (unwritten) style guide, 'law' would be reserved for constraints that, if not strictly true all the time, you generally cannot avoid (e.g. Moore's law), while a principle is a good idea that you would generally be well advised to try to conform to.