Regarding forklifts: I seem to recall (vaguely) that those cylinders are designed for horizontal storage. The relief valve should be designed to contact the vapour space. And isn't there a pin in the frame that ensures they're sitting the right way when they're installed?

For all others, I would say vertical storage only. IIRC, if a propane tank vents liquid, it is venting 250 times more fuel than if it was venting vapour. That's a whole lotta 'boom.'