I'd advise against listening to your dealer.
If the back drops an inch, the front is most likely going up that same amount, which means your weight distribution on the truck has shifted and there isn't as much weight over the front wheels. which in turn means you may not have proper steering if you hit a rogue bump in the road.
I think that made sense.
Basically w/o a wd hitch, any bump that the trailer hits, will push the rear of the truck down and the front of the truck up. A wd hitch is built to prevent this. To distribute the tongue weight of the trailer equally on the truck.
Now, I know you have a half ton truck and are well below the GVWR, but since you have to buy a new hitch anyways to get the sway control, I would buy the rest of the WD part anyways. And you can use it when you upgrade. Cause we all know it will happen.