As a truck owner, I always drive with my tailgate up. However, I've known people who would drive with it down claiming it gave them better gas mileage.  I'm not sure if that's true, but I now know that it's against the law.

The following is stated in Minnesota statute 169.43, which pertains to "Swinging Gate On Truck":

(a) No truck shall be operated on any highway with gate, loading rack, or partition carried in any manner on any part of the exterior of the truck, unless the top and bottom of such gate, loading rack or partition is securely attached to the truck, so as to prevent swinging or becoming loose.

(b) No truck shall be driven or parked on any highway with tailgate or tailboard hanging down or projecting from the vehicle except while such vehicle is being loaded or unloaded, and except when a load on the tailboard renders impossible the closing of the tailboard.

As stated above, the lone exception is if the load you're carrying prohibits you from closing the tailgate.  If that is the case, you must make sure the load is secured as you will be held responsible for any damage, injury or death caused by debris flying out of your truck.

Also, according to Minnesota statute 169.52, if the load of your truck extends four feet or more outside the truck bed, you are legally required to place a flag at the end of it and properly light it when driving at night:

When the load upon any vehicle extends to the rear four feet or more beyond the bed or body of such vehicle there shall be displayed at the extreme rear end of the load, at the times when lighted lamps on vehicles are required in this chapter, a red light or lantern plainly visible from a distance of at least 500 feet to the sides and rear. The light or lantern required under this section shall be in addition to the rear light required upon every vehicle. At any time when no lights are required there shall be displayed at the extreme rear end of such load a red, yellow or orange flag or cloth not less than 16 inches square.

Doing this is crucial to not only protect whatever you're hauling, but to also protect other motorists.