I always declare everything that should be declared.

Apart from taking a few minutes of my time, I don't know why people never like to declare things. In the US a lot of things don't have duty, especially electronics that people like to buy abroad.

As for the whole over $10,000 in currency deal, I think a lot people think the US will confiscate or tax it if they declare it. It's not made very clear on the form. Anyways they just want a record.