Think of it as a triangle with you, the game and the base of tree as the three corners. Gravity acts on your arrow for only the horizontal distance from base of tree to game not the longer distance from you to the game. In your case given it really makes no difference but if you were rifle hunting and the angle was that much or more and the distance was 300 yards or more than you would want to know the true distance that gravity works on your bullet not the straight line distance to target.
Some newer rangefinders take the arc into consideration and give you true distance most don't. The thing you need to remember is that regardless of up or down angle if there is a sharp angle the actual distance will always be less than your RF reads unless you have one that compensates.
For bow hunting unless you are much higher than 20' above your game and unless the distance stretches out to 40 or more yards it's not something to worry about. Same with firearms for the most part if inside of 300 yards and the angle is less than 30 degrees it's not anything to be concerned about.
