the first purpose to having a truck is hauling or towing.
with that said, the rake is there so when you load the box down, it levels the truck out.
so yes, add a leveling kit will make your rear sag like the soggy bottom boys. lol.
personally... a 4" lift would do more, but costs more too.
personally, i plan on adding in roadmasters and then leveling 2". the springs will add an additional 1 1/8" to the rake. so the level will bring up the truck to simulate a bit of a 2" lift. and should be slightly raked or level if i load it down a lot.
keep in mind this is all theory thus far.
Here's the truck with Air Lift airbags and a 2" leveling kit I just installed. Next up is 10 ply BFG T/A KO 275/65/18 tires. Not sure if ill need HD shocks to finish off the touches to make this 150 a pulling machine. We will find out next spring!
Here's our new rig. Nice upgrade from the Shasta tt. HD package handles it great. Thought I might need timbrens for extra bottoming protection but no problem. Just need to move the pin box down to the last hole to get another 1" of bed clearance. Over 11mpg on the 300 mile trip home (up from 9.5 with the tt) and I can feel that it's easier cruising than the travel trailer and much more stable.