Don’t run a Tor node in places that have censorship laws or problems with freedom. In places such as the US and most of Europe it should be totally fine to run a node. What the network really needs is more middle nodes. You can inform your ISP and the local police of what you are doing just to be sure.

The only time you could get into trouble is when you are running a exit node. ISPs and police have mistakenly classified out nodes as local traffic. It is recommended that only organizations such as universities run Tor exit nodes. However, it is important to keep in mind that to my knowledge no one has ever been arrested for running a exit node in a western country.