Rob is absolutely correct........with proper form shoot the horizontal distance to the target everytime.

I shoot about a million shots in 3D competition with hard uphills and downhills all the time. ALWAYS calculate the horizontal distance.

When you are in a treestand and using a laser rangefinder don't click the base of the tree you want to distance.........laser the trunk at eye level.

In reality unless you are extremely high and combined with a downhill shot from an elevated position the difference in yardage is minimal, a yard or 2 at most. Form is much more important than distance relative to angles for most hunting shots.
Also don't blank out when you are aiming. Think top of the heart on exit regardless of shot angle or steepness. This gives you the widest margin for error, will account slightly for those "Mysterious High shots" and will give you some built in cushion if the deer drops at the shot.