All the planes I have ever built have required some right thrust and some downthrust, the amount varied widely. It seems that high wing planes and bipes require more right and down than low wingers do. The amount is best determined by experimenting. when you set up your motor mount, leave provisions for adjusting thrust line by adding washers or spacers at the appropriate places. A good starting point seems to be about 3 degrees right and, on high wingers, 5 degrees down.