If its a bracket that clamps on the tube of the A frame, like a Equal-iz-er hitch, bear in mind that the bars will be pushing down on the bracket with considerable force. If it has vertical movement before tightening it Bracket taller than the tube it fits on, I would be inclined to put it all the way down, because it will probably slip that way as time goes by if you try to mount it up higher.
If this is the bracket for one of the flat anti sway bars that connects to the hitch separate from the weight equalizing bars, I would think you want it nice and level with the trailer frame, but I have no experience with them.
Charles