I usually ignore bad stuff, because I think petitions, outrage and shock is just what the people making the stuff want. I remember when I was 15, Marylin Mason was big, and I thought he was cool just because he seemed to be someone who pushed the envelope and shocked.
Then again, a good bit of outrage is sometimes useful but I'd say mostly when its the endorsement of indecency by an otherwise decent organization, for example the famous Gay Passover debacle that was endorsed by Pepsi.
Is there a hard and fast rule? Not sure, but I'd say in general, more things just need to be ignored and left to fizzle (like Lindsay Lohan as Jesus, give me a break) so it doesn't just attract attention to an otherwise cheap publicity stunt.