@bernie12
I think it honestly depends on the student’s background, and my bias might be showing here. There are two types of students who should do two different things:
A: if you’re a very talented student arriving very well-prepared from a top private or large public high school, you’re going to be well-equipped to push yourself and you don’t need to be conservative with your class selection. Of course the genius kids are going to go in, be very aggressive with what they pursue, and end up at the top places anyway. I mean, HST - those kids are top percentile demigods, of course the rules don’t apply to them. And it was the same pattern here; most of the kids who end up as MSTPs at HMS, Stanford, JHU, UCSF, Penn, etc. were pounding full courseloads of advanced classes from the get-go. But these kids were also maybe Intel/Siemens finalists, were published in high school, had near perfect scores, and got scholarship money thrown at them - they know who they are, and they aren’t coming on CC for course selection advice.
B: on the other hand, if you arrive relatively under-prepared and are a mere mortal, that protectionism is crucial. Sure, by the time you’re an upperclassman you should have adjusted to taking rigorous upper-levels, but that transition period is necessary so you don’t 1). blast an insurmountable crater in your GPA (once you start dipping towards a 3.70 or lower, no matter your course rigor, you’re going to have some major trouble at top places) 2). kill your confidence and momentum, and 3). keep yourself from having enough of a GPA buffer that you’re not utterly desperate to reel in 4.0’s to balance a weak start and can branch out on the actually important stuff (extracurriculars). Having a 3.85 vs. 4.0 isn’t important, but making sure you can survive a tough transition and have a 3.85 instead of a 3.65 is big.
The reason I’m personally biased is that I had a relatively weak high school preparation, bit off more than I could chew, and it was a huge pain to resurrect my GPA enough to the point that I was competitive for the places I wanted to go. By the end I was running with the big dogs course selection-wise and I did receive interviews and acceptances at multiple top-tier institutions listed in your post, but I wished I’d been a little bit more conservative early on (when the classes you’re taking aren’t even that interesting) so I could have taken more risks as an upperclassman when you can do cool stuff (less time collecting 4.0s, more graduate student classes, more time in research, etc). And honestly, while most of the kids accepted to T5 MSTP programs or HST had all of the ridiculous course rigor, MD-only programs at Harvard, Stanford, JHU seemed to care much more heavily about exceptional leadership, experiences, and extracurricular achievement (at least in the cohort that I knew at VU and met during interviews) - giving yourself a buffer to pursue these experiences is big.