when we detail my buddies work truck, we spray down the bed like its no ones business, then we put the tailgate down and spray that off. Salt, sand, and crap tends to sit in the tailgate area the most, we have found.
Seems like one of those soap sprayers would come in handy for doing a truck bed.
I also thoroughly wash-down the bed areas too. Especially, the tailgate section. Depending on the vehicle and what it is being used for, I sometimes even polish and wax all painted surfaces in the bed area too. It's the overall appearance that counts. When it comes to detailing my customers pick-up trucks, I always do whatever it takes.