I used to do this on a regular basis a few years ago, when coupons were less restricted. The hotel/car is not linked to the air reservation, so not showing up at the hotel or rental station will not have any negative impact on your air reservation (this isn't like skipping a leg of your trip). Booking a package can save you a lot of money (make sure to also look at last minute options), and when decent coupons are floating around, there can be some MR-worthy fares.