Either shooting at an upward angle or a downward angle, the point of impact will be higher than if on a horizontal plane. The physics behind this is that when shooting at an angle, either downward or upward, with increasing angle of the shot, the force of gravity has less effect on the drop of the arrow and more effect on the 'speed' of the arrow.
Think of it this way, if you were to shoot an arrow straight down, it would have no 'drop' at all, no matter how far down the arrow traveled. Consequently if you shot an arrow straight up, it would have no 'drop' at all either, however gravity would eventually force the arrow back to earth.
At a distance of 15ft in the tree, shooting out to 20 yds, this effect would probably be negligable.
But suppose you were 30 ft in a tree and a deer was 10 yds out. The actual distance the arrow travels in a straight line is 42 ft, but the horizontal distance is only 10 yds, or 30 ft. In this case, your arrow would drop the same amount as if shooting 10 yds on a horizontal plane, so some compensation may have to be taken into account in a scenario such as this and hold the pin slightly low.
Clear as mud right ?
Best thing is to just practice it!!

chris