If your wing is positive to that line, then you are effectively building in downthrust. In a vertical line the wing has to be at zero or near zero angle of attack. So the incidence angle is your downthrust angle. I've never seen a true 0 - 0 - 0 design work.

It's sort of funny. My first real pattern plane, a Phoenix 6, had no side thrust. Now my current one, the Nuance, has none. Slightly different reasons though!

I agree, everything is a compromise when it comes to trimming. And there are certainly many ways to attack the problem. Please everyone note, I'm agreeing with DaveL!