I believe in Murphy's Law (and that he was an optimist). No matter how hard you try to achieve a "zero" engine offset, 99.99% of the time it will be "up", "down", "left" or "right". With current equipment and AC designs, "up" thrust and "left" thrust are usually worse than "down" thrust and "right" thrust. I'd suggest going with "positive error" and incorporating ~1 degree of both down-thrust and right-thrust.