There is poor advice going on here, once you tell them you have financing they will have less flexibiltiy on the price of the car because the dealerships makes money on the financing.
Better advice is to play dumb and keep your financing in your back pocket for when you meet with the finance guy. Even initially indicate a willingness to use their financing. When with the finance guy ask them to beat your financing terms or offer to go with their financing if they kickback some if the benefit. Even if their terms are lousy, you can immediatly refinance, but still capture the lower cost if the car.