When I set our 2006 Jeep up to tow about three months ago, the jeep-end of the tow-bar was almost three inches higher than the motorhome-end, even with a 6-inch riser. While towing it, I noticed that the Jeep would "wobble" a bit from left to right - nothing violent, but it was a bit unnerving since our other towed vehicle is rock-steady with the same tow-bar.
The Jeep's suspension had been modified for off-roading and it's got 35" tires, so I'm pretty sure those things played some role in causing the wobble. My biggest concern, however, was the height difference between both ends of the tow-bar, so I installed an 8-inch riser and some hitch clamps, on both sides of the riser. Problem solved!
The danger is having the Jeep submarine the rear of the coach during a panic stop if the tow-bar is too low, or having it climb on top of the tow-bar if it's too high. In either case, the auxiliary brakes probably won't be very effective, because all four wheels may not be in contact with the ground.
Kev