Well, securing them is the safest. No matter how you place them if they can roll around they're dangerous.
If I was gonna lay them down in a trunk and not secure, I'd place them in valve first.
My reasoning is I've seen a valve blow off before. It has kinetic energy which means it's going to stop when it hits something (hopefully not me) that'll absorb that energy. Now most likely that back seat cushion won't stop it, as they're just foam and fabric, but the firewall behind the engine might, and it ends there. But that cylinder has stored energy inside. Even if it hits a brick wall, it isn't going to stop. It's going to keep on spinning and bouncing around in that car until it runs out of fuel (compressed gas), beating everything up inside the car.
Recall the fireman that was killed awhile back by his SCBA cylinder? It bounced off all four cement block walls of the bay before it stopped.
With a valve, fate has one chance to get me. With a cylinder, it has as many chances as it can get until the gas is out.
The valve I saw blow off, it went straight up like a bullet, thru the ceiling of the dive shop, and into the place between the ceiling and floor of the second floor. The cylinder kept right on spinning and flying and bouncing. Long after the valve was at rest, the cylinder was still moving and deadly.
Maybe my logic is flawed becuase I've seen what a piece of shrapnel or bullet can do inside an M1 Abrams tank in Iraq. It ricochets all over, shredding everyone inside up until it runs out of energy. If that bullet or shrapnel could exit the tank and not be confined, it'd be a lot less dangerous.
