Propane, Carbon Dioxide are in a liquid inside the cylinder and should only be transported standing up because the pressure safety is designed to work with the gases not in the liquid of the product. In other words, if you transported it laying down the safety could rupture filling the vehicle with the gases. The danger is passing out with CO2 and fire with propane.
Acetylene is very shock sensitive, hence being dissolved in acetone. Do not lay them down. It could cost your life and the vehicle next to you.
Oxygen, nitrogen and argon(and all the mixes of the previously mentioned) are filled with compressed gasses. Think rocket if the valve were to be ruptured. There is between 2205 and 2400 psi depending on the cylinder and the valve has a hole the size of a pencil lead. If you lay it down be sure you know where it could be headed.
There was once a scrapper going through a demolition site after hours and thought the brass valve was too good a find to pass up. He knocked the valve off with a hammer. The 220 cf O2 cylinder flew about a city block and went through a concrete block wall into a retail store. Needless to say he got caught and was prosecuted for theft, vandalism, destruction of property. He was lucky no one was killed.
Many times I was sitting in traffic and would see people flick cigarette butts onto the open bed of my delivery truck. I would roll down the window and ask if they were really that stupid to think they would survive if the truck were to go up. It would take out the whole city block, under the right circumstances.