they used to calibrate how much out of center you should put the ball
one needs to be a bit careful. Three situations for which one should cater ..
(a) back around Vmca, nil slip is not the aim - control is. Unless the POH says different, you want 5 degree bank to the operating engine(s). Vmca is strongly dependent on bank angle
(b) once you have things sorted out and a decent speed for climb (somewhere near blue line, perhaps), then performance becomes critical. Something approximating nil slip will be optimal. As a general guide, looking for 2-3 degrees bank to the operating engine(s)
(c) cruise (and for those aircraft with ADIs not liking small bank angles). The performance benefit of a small bank angle is not warranted so we generally tolerate some extra drag and go for wings level. For awkward ADI aircraft, wings level is used for performance climb as well. Consider that the performance delta is not all that great so pilot workload comes into the equation as well.