Heroku injects a plugin that does this for us, but I just ran into a situation where I had to set it manually. I think it's better to turn it on rather than rely on Heroku.
@gabebw Sorry for the delay getting back to you on this. Since you opened the issue, Rails 4 has come out and Heroku has offered the rails_12factor gem, which is not automatically injected into the environment during deploys, but rather is a choice the developer has to make for their Gemfile. I believe that's the direction we should go and have just opened a pull request for it here:
Nice! I agree that the gem is the way to go here.