on the ground you shoot and you have a arch in the trajectory because of gravity..
In the tree say 20ft up shooting down you are not fighting gravity as much so yes the inpact would be high if you aimed for the same point..so yes you must aim low..and like Bstout said it applies to shooting at an upward angle also to the same degree..
It comes down to practice from the same angle that choose to hunt from..( on the ground or in a tree) on a roof, from a 2nd story deck whatever...
I sight in my 200 lb exomag on the ground first at 20 -30 -40 with my varizone set on 325. Then and I have practiced from up in my tree at 20 ft....I turn my varizon up to 350 and in a tree 2o ft up with my arrow combination Im right on the money..
Now saying that ---I will blow your mind here...what if the deer is right below your stand striaght down shot...You must hold (if you have a varizon) the 40yd mk on him to be dead on...why... because the scope is mounted so high on the xbow...it takes some distance out to allow the scope to be on..If you put the 20yd crosshair on the deer it will hit way low..Idealy wait till the deer gets out away from you a bit and take a better shot than straight down..
After all the time money and effort you go thru to get yourself to the woods and in a situation to take a shot...dont forget that last most important thing..PRACTICE...