That all depends on where your current situation (single, married, kids, etc.), where you live, what the disaster is, the evacuation points/routes, weather/climate, etc.
I would be responsible for my family, so my bike would probably be one of the last things I packed up, though I would seriously consider taking it (you never know, it may be easier to ride than walk). Of course, if you have a boat, that might be more important, depending on the situation.
Contact your local Red Cross/Crescent and your local town/city/state/province/etc. government and see what the evacuation plans are for where you live. There should be contingency plans in place for emergencies and emergency routes, relocation centers, etc. should all be in made accessible to the local population. (At least I would hope so!)