I'll attempt an explanation based on some physics. If you don't want to read all the numbers, the short answer is that shooting down will cause your arrow to hit high, but we are only talking an inch or two. So for all practical purposes, you don't need to aim any different in most situations.

Lets say you sight your bow at 20 yards on flat ground. Let's use an arrow speed of 200ft/sec. Now use the equation time = distance/velocity. So time = 60 feet/200 which means your arrow reaches the target in 0.3 seconds. Now lets use another equation which shows how much your arrow drops during that 0.3 seconds. distance(feet) = 0.5 X 32 X time X time (32ft/sec2 is the acceleration due to gravity). So distance = 0.5 X 32 X 0.3 X 0.3 which equals 1.44 feet. So to hit your target, you aim 1.44 feet high (but your pin is right on the target)

Now lets get into a tree stand that is 20 feet high and aim at a target that is 60 feet away (line of sight). To calculate what angle from horizontal you are shooting at, is arcsin 20/60 = 19.47 degrees. When you were shooting on a horizontal, gravity had it's maximum affect on the trajectory of the arrow (perpendicular to travel path). Since you are now shooting at an angle down, gravity has less affect on trajectory and more affect on increased arrow speed. When on a horizontal, your arrow dropped 1.44 feet, but now at an angle of 19.47 degrees down, it only drops cosine19.47 X 1.44 = 1.36 feet. The difference between 1.44 and 1.36 is 0.08 feet, or 0.96 inches. So if you put the pin that you used to sight at 60 horizontal feet on your target, you will hit about 1 inch high. The higher your stand is, the bigger the difference in arrow drop compared to a horizontal shot. This is why it's really not an issue, unless you like shooting from 30 feet or higher.