It's a high-trust environment, and I think very few judges would demand an official transcript. What's interesting is the risk-versus reward here. You can get disbarred many years into a successful career if this came to light. Meanwhile, clerking for a judge is mostly resume polishing. It's very educational, but you end up working the same job after your one year appointment, you just get to signal to your lawyer friends you had really good grades in school. To Martoma, that was apparently worth risking his career for an indefinite period into the future.
That's exactly right. I can report that I've had five legal jobs since I started law school (including two clerkships) and nobody has ever asked for my official transcript. I'm frankly very surprised that somebody noticed the alteration.
That being said, it still takes serious guts to pass off an altered transcript. He surely had no way of knowing that the judges would not eventually require an official transcript. I certainly assumed that I would need one before I could be formally offered the job.