First of all, I have never seen a dedicate, " nothing to declare" , line going through Customs entering the US. They are common in Europe and some Countries in Asia. I have always gone through a single/double line to hand over the Customs form or to have a quick talk.

As for declaring, item 15 states to place a value of the items that you have purchased overseas. Personal items you brought from the US to overseas and back again to the US do not need to be declared.

http://www.cbp.gov/xp/cgov/PrintMe.xml?xml=$/content/travel/pleasure_vacation/sample_declaration_form_.ctt&location=/travel/vacation/sample_declaration_form.xml