I like to have my tandem as close to level as possible. Sure, nothing's perfect, but to my mind all the more reason to start out as level as you can reasonably be. When I switched vehicles I had the same issue. I had it all worked out just right on my previous rig, but the ball height was not exactly the same on the new rig, plus there was the variable of how much the tongue weight would push the hitch ball down (and to top it off, more distance from rear axle to hitch ball thus more leverage).
What I did was set it up with the old gear. This showed the ball to be around 1" low from where it would need to be to have the trailer frame level to the ground. I then had two choices. One was to use a drawbar with an additional 1" rise; the other was to use the same drawbar but with a ball that had a one inch rise built into the base. Since I already had two drawbars, I was able to just buy the ball with the 1" rise and have a few different combinations to choose from.
The winner turned out to be the lower drawbar with the 1" rise ball. Either would have worked though (but there were other factors that made the ball riser slightly better). I think when you are only talking one or two inches, the tongue weight (and hence the "press down" of your tow rig) is not going to change that much, so you can probably just add what you need. Of course if it doesn't turn out just right, then adjust.
To set it all up I parked in a Home Depot parking lot (in the back forty) that was level and had good pavement - so I could measure pretty well.
Ultimately, I "re-tested" by checking my four hubs with an infrared thermometer, looking to see them all similar (to me if one axle had been running hotter, then likely that axle would have been getting more weight). It all turned out fine, and almost took longer to write about than to do.
I can't see changing the actual hitch. You may haul a variety of trailers, or something might change, and that is commonly accommodated by varying drawbars and/or hitch balls.
