Top Secret is only one of several levels of classification.

You really need to mark things as unclassified in the government because other people need to be able to answer basic questions like:

- what computer am I allowed to view/edit/save/transmit this on?

- can I talk to my coworkers about this, and if so, which ones?

- what do I do with printed copies of this document?

Answering these questions wrong can land you in serious trouble, including jail time. So even as a courtesy to your peers, you want to mark them (U).