In a high wing plane with a lifting airfoil, some down thrust is used to keep the plane from climbing a lot when the power is increased. On a low wing plane, down thrust could cause the model to lower the nose when power is increased, though to make this point clear, you would need a huge amount of down thrust. Now if you are asking about a plane with a symetrical airfoil and the thrust line and wing are fairly well lined up [like on a typical sport plane], no down or up thrust is needed. Some right thrust might be good as a fine tuning tool, but this can always be played with after you feel the plane out. Those black plastic engine mounts are easy to hand work any angle you want by filing away material on the back side where it mates to the firewall. Final thrust angles are easy to check if you bolt a long scrap of something straight in place of the prop and hang an angle finder to it after you have leveled up the plane / wing / stab, or whatever you are setting this angle in reference to. Setting the correct angle isn't brain surgery and the exact angles aren't that critical on a 60-70 mph model, it is done for convenience to favor a certain mode of flight, but it can hinder the way the plane behaves in other modes of flight.