RE: downhill shooting
Right you are Buzeye. If you know the true horizontal distance to the target that is all you need to use for the shot. For bowhunting, if you note the distance between the base of the tree your stand is in to some common landmarks (green bush, oak tree, big rock) and know that distance when the deer walks by you can just use that distance for the shot.
Since I'm a rifle hunter I'm more concerned about shots over 200 yards at slopes from 30 degrees and greater. Usually it's only a couple inches but a 200+ yard shot on a 45 degree slope can be more significant so I try to think in terms of that on these kind of questions. If the slope is pretty good I usually try to figure the amount of slope and compensate accordingly.