There are plenty of instances where it is illegal, in China, Iran, Syria. Even if no law exists on the books, they may arbitrarily imprison or torture you for using Tor. However, I've never heard of someone in a Western country being prosecuted for merely using Tor. Yes, the threat model for us is to avoid leaving traces of Tor use that help to build a case against us. You could save some money by DBANing it and reinstalling the OS. http://dban.org