Thanks for the responses, but I probably didn't explain well-enough what I wanted to do. A plumber was doing some work at my place of employment, so I explained what I wanted to do, and asked the same question. His response was simply to: 1)shut the gas off at the meter, 2)turn on the gas for the furthest appliance, 3)cut-away - If 1 or 2 doesn't work, DO NOT PROCEED TO 3! He said the same thing about trusting a valve, but he said it would probalby close around 99.9%, so the little gas left wouldn't cause a problem. Furthermore, whenever cutting steel, one usually uses oil, so the risk of spark is quite low.
Like anything else, don't always believe everything you hear, so I plan on checking with a couple other plumbers before making the final cut!