A cylinder is stronger sitting on its end. If it did flex by being on its side and bouncing down the road, any flex could compromise the glass lining. Same reason they tell you to not ship glass flat, or lay your flat panel display down flat while shipping...it puts a lot of stress on things.
For a practical situation, as long as you aren' stacking stuff on it, you should probably be okay. In a trailer from the factory, if they stacked them laying flat, most would likely be ruined while on the way to the retailer.