Natural gas and propane will not ignite unless a stociometric volume of both gas and oxygen are combined. The gas line itself does not contain oxygen. When you cut a gas line you introduce oxygen as you penetrate the pipe, but with the gas off there is not enough gas and air mixture in the proximity of a saw cut to ignite. BUT you should check with your local code to determine if a union is legal in that situation. In some cases, unions are not allowed in basements. Are you tying in at an existing connection and replacing the connection with a Tee? Have you checked the gas line for existing Tees and plugs where you can more easily take-off with a new run?