NSA does recommend that their tow bar/brake system is set up as close to level as possible, and it is good to try and get it close to level. Drop bars are the way to achieve that.
However, know that drop bars come with other downsides. They increase the torsion load on your hitch receiver (due to extra length), and the more of a 'drop' you use, the more ground clearance you take away back there and leave it more prone to scraping. When you back up over any kind of incline, or pull out from gas stations or whatever that has an incline....The bottom of your drop bar is usually the first thing that will scrape. Too many hard hits or scraps can weaken it, leading to risk of it failing.
The less of a drop bar you can use back there, the better off you are in terms of risk of scraping. But, then, especially if using a Readybrake system, the more off level the tow bar is, the less effective your braking will be. You just have to find a 'happy middle ground', and pick a drop hitch that gets you fairly close to level without causing too much scraping.
When I towed a Ford Fusion, I used a 4" drop. That thing scraped, a LOT. When I took the 4" drop bar off, the scrapes underneath it looked really bad. After the Fusion was totaled in a wreck (bout year and a half ago) and I replaced it with a Ford Taurus, fortunately the Taurus height was a little higher, so I went to just a 2" drop. Much less scrapes now.