The US doesn't have "Nothing to declare" lines. You fill out the customs card no matter what.

That being said we have always just put a dollar amount (we've never even gotten close to the limit) and that has always just been rounded off at that. Only once has it been questioned at all, the one day we got directed through the foreigner lines rather than the citizen lines. (The foreigner lines were almost empty, they started feeding some of us into part of the non-citizen area.) The guy seemed to have a very hard time with the concept that I wouldn't consider buying electronics in China--I think he thought I was being evasive when in reality I was just tired.