Actually if you shoot up hill you should aim higher and if you are shooting downhill aim lower. All has to do with gravity. Shooting uphill the bullet is going up against gravity so the bullet will slow down faster and if you are shooting downhill you are shooting with the force of gravity so the bullet has less resistence
Actually you are totally wrong about shooting uphill.The bullet will strike higher.Gravity is always acting on the bullet whether it is travelling uphill or downhill.The reason the bullet will strike higher in both cases is becausegravity acts on the bullet for the horizontal distance that it travels.With a steep angle the horizontal distance will be less thana straightline to the target.The rangefinder measures a straight line to the target.