The circumstances under which a CO2 cylinder will 'blow' are pretty severe. Severe enough that you'll have a host of other problems long before you see the tank rupture.
However there are 2 very real 'threats': System leak and valve compromise.
Systemleak is prelatively simple to prevent. Use a spray bottle of soapy water to find out if any of your joints or junctions leak. If you are really concerned, make this a regular maintence task. Eveyr month or every time you clean the fridge or eveyr time you empty a keg--- whatever best suits your needs. Or do as suggested: turn off the tank at the main value between uses. Otherwise, jsut make sure you've got no leaks and then don't worry about it unless you change yoru system.
Valve compromise is the other danger. Basically the weakest point on a CO2 tank is the valve. This is made more dangerous when you attach your regulator. If the tank falls over or the valve (or anything attacked to the valve) is struck hard enough, the valve could be compromised. This woudl result in a massive rush of air as the compressed CO2 escapes. This rush can cause the tank to be propelled like a jet, of sorts. Therein lies the danger. This is easily prevented by buying a gage cage and by securing the tank to another structure using a tank strap (pretty cheap at a welding store or farm supply place). Keeping the tank inside the fridge further protects this.
Of these 2 risks, the first is more likely than the second. It wodul take a pretty hard blow or a really freak accident for you to strike that valve hard enough to compromise it.
