So, you think the valve just sits still? If so, you need to brush up on your basic physics.
Of course, getting hit by a valve is probably more survivable than getting hit by a tank. But both are going spoil your day....
Where a proper cylinder cage isn't available, I've always laid my cylinders with the valve facing rear. In the event of a forward collision, the valves won't get impacted if the cylinders slide forwards. In most vehicles I've used, it has been impractical to store the cylinders sideways.
No, I sure the valve moves.
What I meant to say was that it's the tank that's going to do most of the moving. The valve will shoot off and cause some major damage, but not as much as the tank.
I tried to clarify this in my other posts on the first page =]