It is extremely important to store your spare tank properly.
1. Store it outside (NEVER in an enclosed area, such as a garage or shed)
2. NEVER keep your tank in your car trunk, if you do need to transport your tank, put it in the backseat with your windows open.
3. ALWAYS keep your tank away from a direct heat source, and NEVER keep your spare tank under or near your grill.
The best place to keep a spare tank is on your deck or the side of the house. If any gas happens to leak, it will dissipate. If your tank is exposed to the elements, it may rust a little and the tank may start to look "aged", but the gas won't go bad.