You can't break gears themselves by screwing the grub screws too far into the gearbox. The grub screws are pressing down on the outer ring gear and if they are in too far, the ring can be pushed out the back of the gearbox, breaking the motor mounting plate along the way. What Rory is doing is the optimal way to set the grub screws up. If your grub screws have a point on the end, filing it flat will tighten things up a bit.