Originally Posted by 99_TJ_Wyoming
It is meant to be "as flat as possible", this isnt always possible, I would say that as long as the angle isnt steeper than about 10 or 15 degrees you should be ok, just keep in mind that stopping and accelerating will put undue stress on the towbar at the ball, more so when stopping.
15 degrees sounds a little high - you should check with your tow bar manufacturer. The manual for my Roadmaster Sterling says no more than a 3" height difference between the center of the receiver and the center of the pins, which is a in the 5-10 degree range.
But that is easily solved with a drop receiver (flipped over if necessary).