FAR 23/25, model aircraft, whatever .. all same same
Not always true!
The flight test guide does permit up to 5 degrees bank angle for use during performance testing, which reduces the rudder angle required to keep the aircraft straight, thereby reducing drag in the OEI situation,
generally speaking.
However, the performance benefit is certainly variable according to the aircraft's aerodynamic configuration, i.e. if the aircraft is equipped with spoilers for lateral control, some aircraft may, or may not gain a performance advantage with 5 degrees of bank, depending on control wheel deadband angle characteristics when out of center.
The aero guys will fine tune control wheel deadband during flight test according to the actual characteristics desired, before freezing the production configuration. Pilot Induced Oscillation (PIO) sometimes becomes an issue here.
This may explain why the 777 calls for zero bank on climb out with OEI (doubtless helped by the aircraft's relatively high power to weight ratio compared with older designs).