Not only is it a bad idea it's against X plan rules. You must disclose X plan before negotiating. This is only fair to the dealer. If they want to beat X plan they can try but with X plan you get every possible incentive (even dealer cash) and you're protected from high doc fees and add-ons. Keep in mind the dealer gets an additional $400 or so from Ford on the sale so if they're forgoing that they're probably making it up somewhere else (trade-in e.g.).
Start with X plan price and negotiate from there.