The problem with up and down hill becomes the vectors. With shooting straight, all energy from the projectile will be acting on the X plane, with gravity pulling down on the Y plane. That's what a scope is generally set for. However, if shooting downhill, you've got a portion of the energy acting on the X plane (not as much as normal), and then some pushing the bullet down on the Y plane. This will cause the bullet to appear to drop a little faster, requiring a bit of hold over.
With shooting uphill, you've got some of the energy pushing the bullet higher on the Y plane, acting AGAINST gravity (when shooting flat the bullet does not work against gravity as the X and Y vectors are independent), requiring you to shoot just a wee bit lower than normal.
Now, practically, you can likely hold dead on and still make a killing shot, but for the sake of argument those would be the corrections needed.