Unless you manage to squeeze out the lowest price (see the Kia 2015 thread) and have finance ON TOP on that. Then it's free borrowed money :P
But unless it's a special event/sale, dealers will usually not give you the lowest price when going finance. One tactic would be to say you are paying cash and squeeze the dealer the lowest price (invoice price if possible by using unhaggle free report) and then say you will lock the deal if they give you a 0% financing or you walk away. Ofc you must make sure you haven't already emotionally attached to the car.