Hobbyman speaks the truth. Risking a melted/burned-through piston is not worth the buck or two a tank you're going to save (IMHO). Pre-ignition (a.k.a. pinging) is a very bad thing and will eventually ruin your motor.

I would tend to trust the word of the guys that designed the engine over the word of your dealer. If he says its OK to burn cheap gas in your Crossfire I would ask him to quote a technical reference from DC that says its OK to do so.