Several considerations -
(a) at much lower than normal speeds (at/near Vmc) one needs to crank in some bank (certification limit is 5 degrees) towards the operating engine(s) to achieve expected handling characteristics. Achieved Vmc is very bank angle dependent. Climb performance is not the name of the game at this sort of speed .. staying right way up is. Indeed, there may well be no climb in this speed region .. the aircraft often will be in a descent.
(b) once the OEI situation is sorted out and the speed is up at/near the optimum climb speed, best climb performance is achieved with 2-3 degrees for most aircraft. This is a fairly demanding flying exercise for other than a short period. Hence one normally trims out the control loads to suit.
(c) some attitude systems don't like small bank angles so the technique is to fly wings level and accept a small loss in climb performance. Alternatively, if terrain clearance is not super critical on the day, the pilot probably will prefer to fly wings level due to the reduced workload. Some aircraft will have spoiler deployment with other than a "level" control column so that is a driving concern, for such Types, towards flying wings level.
(d) at higher speeds, the need for bank is minimal and one normally chooses to fly wings level.
(e) flying straight with any bank angle, you won't have a centred ball .. it will be displaced somewhat towards the low side. Conversely, wings level, the ball should be centred as there will be no force to displace the ball from centre.
Piston, jet, glider, rubber band powered model aircraft ... makes no difference to the ball situation.
Pitch angle will be appropriate to speed, thrust and Type.