An extra five gallons is not near as explosive as the fumes in a empty gas tank. However, that is a moot point.
If I am in an accident strong enough to crumple, pierce and ignited the gas can secured in my truck bed, I have other worries. Spare gas, is like the ability to change your TV and TT tires. It is the difference between an annoying twenty minute delay and sitting on some back road waiting for road service to find you. That also assumes you have cell service to make the call.
I prefer to have the gear in need. That includes gas, jacks and the tools needed to use them. Others prefer to wait and have the pros do it.