I think you have to plan for "possibilities" based on their liklihood.
Greater chance you will lose your job, get lost in the mountains of Oregon (especially you Californians

) , or be faced with no electricty, hurricane, or an ice storm, or maybe Zombies.
I plan, best I can, to handle the certainties first: Snow storms, Thunderstorms, 24 to 72 hour power outage, etc.
Then the "not so certain" but plausible scenarios: Hurricane comes up East Coast, House burns down, lost in unfamilair territory, criminal activity.
Lastly, and much more unlikely, are those things I would count in with the big "Biblical" events:
Zombie Plague, Alien invasion , a shift in the earth's poles, Flying monkeys.
While I think an asteroid strike is likely in the next 10,000 years, I'll play the good odds that I won't have to deal with it. Or, in other words, I'm not building an ASS (Asteroid Survival Shelter).
If they announced a 1 mile across Asteroid, with a 90% certainty of hitting the earth in one year, do you know what kind of chaos and pandemonium would ensue? It might be easier surviving the aftermath, than the 365 days waiting for it.