According to what I've read so far (I'm not a doctor) the way I understand it is that keeping Potassium iodide is like keeping activated charcoal and syrup of ipecac: it's important to know the risks and when to take them. If you stock KI (potassium iodade) I'd say it's as much important to have a radio available to listen to emergency officials and the poison center phone number readily available because there's a risk of allergic reaction. I guess there's no concensus as to if everyone should stock some. It's probably a good idea to have some because in order to be effective it must be taken within 3 to 4 hours following exposure.
On the other hand KI only protects the thyroid gland against radioactive iodine and would be ineffective against other radioactive isotopes. In the case of a dirty bomb, radioactive iodine is unlikely because it's tough to obtain from well garded nuclear plants. It's easier to get low grade radiation material that doesn't release radioactive iodine from places like hospitals. On the other hand U.S. forces found diagrams of American nuclear power plants in Afghanistan along with training manuals that listed nuclear power plants as among the best targets to spread fear in the US. But they would have to use a large commercial airline crash to release radioactive stuff because the containment walls are made of concrete 2-5 feet thick with a steel lining...
In short I'm not an expert and can't give any recommendation but personally I would get some but I would also read as much documentation as possible such as this:
http://www.thyroid.org/professionals/publications/documents/thyrosafe.pdfOther preparations could include knowing your blood type in case blood transfusions are necessary; having a family emergency plan that your whole family is aware of since phones and computers may not work after a real nuclear bomb detonation because of electromagnetic pulse, etc.
Good luck
Franky