In a rear-end collision, it is very unlikely for the tank to explode, but it might become a 40 lb projectile. Might be best to have a tight mount to keep it from moving under normal driving, then a chain/cable to reduce chance of theft and keep it from flying.
IMHO, the most common dangerous why they are carried is in a SUV or Minivan. Any accident or even hard stop can have that 40 lbs flying among the passengers. Plus it is easy to think of the tank warming to the point the relief valve dumps gas into passenger compartment.
There are reasons for regulations requiring vents to any compartment or stored outside.