Read Drew's post. He's correct in that the newer transmissions have their shift patterns altered by the tow/haul mode. The older transmissions (like the one the OP has, the E4OD) the tow/haul mode only locks out the top gear. So the tow/haul mode will not improve his mpg, in fact it will most likely make it worse. It will eliminate down shifts though, but only because it never upshifted.
As to whether or not you should use tow/haul mode, well that depends on terrain and whether or not you are putting strain on the transmission. I've hauled extremely heavy loads and not had to use it. Then I've hauled light loads and needed it. If you transmission is cycling through the gears a lot and seems to be hunting for the right gear, put it in tow/haul. If it's not doing that then it's not needed.
