Based on my travel experience (2 trips to North America, 1 to the Middle East), nobody really cares about the contents of your laptop. Come on, it's not 1998, pirating stuff over the internet is a lot easier than bothering to carry it physically.
What customs are usually interested about is
1) Large quantities of identical stuff which may be contraband
2) Illegal items, which oddly enough includes most food. Also drugs, firearms, etc.
Security may check your laptop to detect any unaccounted cavities which can carry contraband or explosives - my laptop was coming apart and security looked really concerned and ran it through quite a few extra tests.
Customs may want to check any software you may be importing for sale, but the nature of your visit must hint you may be carrying such stuff - like being a contractor who is visiting the US to install & configure software.
If you're just a tourist they're not going to look, not even in random searches. It's difficult to determine or prove the files are not legal under US law right in the airport. You should only be concerned if you were previously convicted for piracy or have a strong reason for having your laptop searched, like being a spy, terrorist or Julian Assange.