The engine is mounted off to one side so that the spinner can line up better with the front of the cowl. It won't make any difference with how the plane performs, it is done for appearance. I've never run into a 1/2A design that needed right thrust to fly straight, you will see the need in larger models, though. Some guys indicate a tiny bit of right, just so a tiny bit of left thrust doesn't get accidently built in. I would rather just build straight and get my impressions about what the plane needs after flying it. Up/down thrust angles are far more important to pay attention to, these make much more difference.