I think it has a lot more to do with form than the Pythagorean theorem (A squared + B squared = C squared). For the purposes to shooting from a treestand on flat ground, A squared is the distance from the deer to the base of the tree, B squared is the hight of the teestand, and C squared is the distance between you and the deer itself.
We all know C squared is the actual distance of the shot, but A squared is the trajectory. So let's assume you're 20 feet up and the deer is 15 yards (45 feet) away.
A squared = 45x45 = 2,025 feet.
B squared = 20x20 = 400 feet
C squared = 2,425 feet
Which means C = 49.24 feet, or 16.4 yards.
That's only a difference of about a yard and a half at 15 yards. When we crunch the numbers for shots out from 20 feet up and 10 yards out, the difference between shot and trajectory is only 2 yards. At 20 yards, the difference is only a yard. I've shot a lot of slow bows over the years -- 140 fps and less -- and I can't see where even shooting 10 yards for 12 yards would cause anything other than a slightly high lung hit on a deer.
If you're taking high angle shots in the mountains, then yes, the Pythagorean theorem can definately come into play. But out of a treestand on level ground? The difference isn't enough to worry about. Concentrate on maintaining consistent form (i.e. bend at the waist) and it's not an issue.