Power

Uphill or downhill you will always need to aim lower than on level ground. The difference depends on 3 things-projectile speed, angle of slope, and distance to object. If you have a very fast projectile shooting on a slight angle and a relatively close object the difference is not enough to worry about. Change any of those 3 things (like a 45 degree slope or out to 300 yards with rifle) and you need to start compensating for things.

Best thing to do is set up a practice area and take some shots similar to what you might find in the wild.