With drum brakes, I have always tightened them till I can't turn the wheel, then backed off till the pads are just rubbing the drum. My Dad taught me that, he said that tightening them all the way up gets the pads aligned with the drums...I have no idea if you actually have to do that or not. You should adjust them though, it will help to make sure all the wheels are stopping more or less evenly and keep the truck from pulling one way or the other under heavy braking.....
