All the forces acting over any airplane make it rotate around the CG, if they have a combined lever respect to that point (thing spatial point here).
Undesired rotations is what the down and right deviation of the thrust achieve for tractor configuration.
Down thrust helps reducing or eliminating the lever respect to CGs that are high, so the airplane doesn't show a nose up tendency with power application.
Right thrust helps increasing the lever respect to the CGs (located over the center line, if the plane is laterally balanced). By doing that, the designer introduces a yaw moment or torque (to the right) that compensates for the left yaw moment associated to the P-factor and gyroscopic precession of the propeller, so the airplane doesn't show a nose left tendency with power application, especially for tail draggers.
http://en.wikipedia.org/wiki/P-factor
http://en.wikipedia.org/wiki/Precession
Consider how those forces could affect your design for deciding on any thrust angle.
If yours is a trike, you will not need much, if any, left thrust.
If your engine is about the same level than both wings, you will need zero upthrust.