I've always taught and was taught to keep the ball centered.
you can do that wings level - has both advantages and disadvantages. Important thing is what is most critical at the time in the circumstances ?
In addition, systems limitations may have an input - eg some ADIs don't like sustained small bank angles.
the ball was kept half out and a bank of 5 degrees towards the good engine was needed.
appropriate for OEI back around Vmca
This would keep a yaw string (if installed) right down the centerline of the aircraft.
don't think so ...
His thinking is that airliners are no different.
just bigger and (usually) noisier. Jet versus prop has some effects due to prop wash