- If you are trying to save the user mouse-clicks.
If you don't care because the form isn't complex enough or you don't have the time or resources to define and implement default values, then don't bother.
- If the user of the system would most likely select the option chosen.
If 80% of your audience is U.S., then make U.S. the default with the option to change. (Don't, however, put U.S. on the top, if you care about having an equitable, global form.)
- If the user is not being asked to make a declaration for legal or compliance purposes.
Don't default them to the "correct" answer if you want them to make an explicit electronic declaration. Don't default them to the "incorrect" answer, either to make them pick the "correct" answer. A recent usability test I conducted on just such a page showed us that users expect a defaulted answer to be the "correct" answer.
- If the default option is the "right" option.
See 3. above.
Additionally, user expectations would be that, if you are providing default values for some controls in a form, you would provide default values for most similar controls in the form (for example, all yes/no combinations or all dropdown selectors would behave similarly).