The OEM gearing is already undercut. Basically wider at the end of the dog with narrower at the base. This is done to make the gears want to pull together and hold.
When we undercut we increase the angle and also do a more precision matching of the dogs to the mating gear. So, the load is held more evenly and not having one dog doing the bulk of the work until it wears enough that the second one can start helping. The OEM undercut is done with some slop so that it will fit any other mating gear that they pull from the box.
You can't also just go cutting on the dogs. If you cut too deep to fix a worn dog you can cut through the "crust" where it's hard into a softer base metal. This means the nice undercut you have won't last long. In those cases where we'd have that issue we have to hard weld up the dog and then do the undercut. On average we have to do this to 2-3 of the gears in any given trans job.