Look at the instructions, they will tell you the bars are to be parallel - and there's good reason for it.
Also look at the actual cam and how it fits into that upside down "V" in the spring bar. This is how the WDH keeps the trailer straight behind you. When the trailer moves away from straight behind the truck, that spring action of the cam climbing out of the V resists movement. When you have it at an angle it makes it too easy for the cam to come out of the V and you have diminished sway control. The steeper you have the bar, the easier it is for the cam to move out of the V. That defeats the "active" sway control.