If the dissident could approach you to ask for your help, he wouldn't need to. You have to gamble on opening up that channel for anyone to use.
You have to pick your Blackstone Number. If the actual ratio of criminal to innocent exceeds it, you should not participate in Tor. Otherwise, you should.
As for myself, I don't think I'd go as high as 10, but I might be willing to enable as many as 3 people to anonymously commit the worst information-based crime I can conceive without my knowledge to enable one person to achieve the greatest assistance possible from safe and unfettered access to information.
Unfortunately, if I run a Tor exit node, I am likely to experience government persecution as the identifiable scapegoat for all that criminal activity that I was willing to tolerate for the sake of helping one person in need. By prosecuting exit node operators for traffic passing through, government policy effectively sets the Blackstone Number for everyone to zero, and damns the innocent.