What is the purpose of downthrust?
With regards to John's comment, we should remember the other player applying torque to the Center of Gravity, the horizontal stabilizer/ elevator/ stabilator/ trim tab.  Ideally both the thrust line and the horizontal stabilizer will be 0 degrees to the horizon with the wing at optimal cruise AOA.
Under power, relaxing static stability with a little competing downthrust will lessen the need for excessive trimming any time thrust is changed, and also leaves static stability full on when the plane powers down to land.
This is a very safe and widely used application of downthrust for GA aircraft.  Because the cosine of the small downthrust angle is very small (cos 3 degrees = 0.9986), effect on the horizontal thrust component is minimal.