I read that the elimination of pressure in a propane line will destroy the pipe sealing compound at the treads, causing leaks. That is the reason a propane seller will require a leak test if the tank is allowed to run out.
During normal usage, a propane plumbing system is at a constant pressure. This means that as long as the tank has gas and is supplying the system with propane, a constant pressure is exerted on the piping and the pipe joint compound. The pipe joint compound will expand during normal pressurized usage and will retract if the system loses pressure. This loss of gas pressure may cause leaks to form because of the expansion and retraction of the piping compound within the propane plumbing system.
Will allowing the pressure to drop to zero also destroy a pipe joint seal where Teflon tape was used? I am currently using small portable tanks connected to gas logs by a 1/2 steel line where I used Teflon tape on the joints.