For the curious here is the reason why some gear manufacturers may require a break in period, and perhaps some may not.

The only reason why any machine that does not operate at a wide range of temps would need a break in period is for contact surface mating. For the same reason cam lobes & lifters and piston rings & cylinder walls break in, depending on the machined contact surfaces of the gear it may require a simliar mating procedure.

If the manufacturer uses a machining and preparation process that leaves the gear contact surfaces microscopically smooth, then no break in would be required.

If the machining process leaves the surface slightly rough at a microscopic level, a certain amount of break in will be required to mate the contact surfaces.

Concidering the forces that those gears have to go through I say do whatever the manufacturer recommends for their product.