That depends to some degree on where you live, and the level of risk you are willing to take.What's a higher likelihood...armed robbery or something like Sandy?
While tornados may not be unheard of in my area, I'm not building (read: digging) a storm shelter - my bigger concern is wildfires. But a mere 10miles from here flooding could be a bigger concern.
While loss of electricity can be a problem (esp in July!), currently the risk seems low enough that I have not purchased a backup generator, though I sometimes consider it - to the point where I've researched propane vs gas powered.