Mine go sideways or valve to the front in the tunk, usually vertical in my truck.
Accidents do happen. In my trunk, if the car gets hit from the rear it won't take a big hit to nail any valves pointed to the rear. The valve may not come all the way off, but a full tank if essentially a gas powered gizmo. If it does break off or cracks, the gas is coming out at the point & will drive the tank in the opposite direction.
The valve has no propellant. The only danger from it is immediate, when/if it breaks away clean. After that it's simply inertia. While that does pose a potential hazard, the tank is under power until it loses enough pressure to no longer move it. It poses a hazard for a greater amount of time & distance.