It definitely depends on the angle of the shot/how steep the hill is. If your 18yds higher than the target and the target is 30yds away then I would just hold the 30yd pin on what you want to hit. However, if that hill is 35ft high and you're shooting at 30yds then putting the 25yd pin on where you want to hit would give a slightly better result than using the 30yd pin. Like others have said, correct form is crucial when shooting steep angled shots up or downhill, but what you are doing when you are shooting angled shots is compensating for the difference in how gravity affects an arrow and it's "drop" considering how you have "compensated" for it on level ground already with your sight.