Last things first: on my N2 tank I use a regulator/flowmeter that is officially for argon/CO2 and I don't worry about it too much. I have no doubt there is a minor difference in precision, but for what we're using it for I'm not so worried that it's worth the headache of running sums like that. I, too, have spent my life in the arts, not maths.
A commercial cylinder shouldn't leak. Not only are leaks expensive to companies who need to use their gas to turn a profit, but gas leaks are, in general, dangerous, and the companies who fill the cylinders could be held liable if they filled a tank with a faulty valve. I know from working with the fire department, valves are rigorously certified, checked, and re-certified on a schedule, by the filling companies. If you want a simple test, you can seal a flap of turkey bag tight across the valve outlet --just bind it in place over the threads with a few hard turns of twine-- and see whether it bulges. No bulge, no leak. And if the leak is too slow to show like that, it's too slow to cause you any brain damage. Helium would float away, yes, but Ar and N2 are both significant componants of the air you're breathing anyway, so at that microscopically higher percentage, you'd never notice.