ORIGINAL: bigbulls
if you are 30 feet up in a tree and the deer is 20 yards (60 feet)out from the base of the tree your straight line distance from you to the deer is 22.3 yards. If you shoot for 22 yards you will hit high. On a steep angle like this 2 yards can make a difference in a great hit and a marginal hit.
10 Yards up and 20 yards out isn't a very steep angle. You're talking about adjusting your aim to compensate for 1 foot of distance. Now think about the reduced effect of gravity on your arrow. Your arrow is now travelling in almost the same direction that gravity is pushing it (unlike a shot from the ground). If you're thinking about adjusting your aim for 1 foot you're thinking too much.
Like Bigbulls said though, the shorter the shot (or the sharper the angle), the more adjustment will be needed. I've found that if I'm between 15 and 20 feet up in a tree I don't need to worry about adjusting my aim until it's under 15 yards. Then I aim a bit low.