As a warning, I'm at the end of my MD/PhD, and I'm quite jaded despite having a phenomenal PhD mentor.
The pre-M1 rotation is not a throw-away rotation. There are definitely people who do a rotation, fall in love with the lab, do another rotation in another lab just to satisfy requirements, and join. I think it's still worthwhile to do at least two because maybe you'll like somewhere else better. One of the reasons it's often wasted is that people don't get really quality advising on who to rotate with pre-M1. After you've been at the institution for a while, you get unofficial mentors who can steer you towards the type of PI you want. For me, I didn't like rotations 1 or 2, but 3 was a good fit. If you want to speed the process along, seek out those mentors early. Your MD/PhD director may be that person, but it's always good to get advice from someone who has no vested interest in your career.
I personally think that as soon as you step foot on campus, you should find an experienced faculty member who truly cares about students and ask him/her to be your adviser to discuss rotations, difficulties in lab, making a committee, etc. Your PI should be your best advocate, but it's always good to get outside advice. You need to find someone who will tell you, "That PI is known for being awesome / not being around / mistreating / firing their students... that thesis committee member is impossible to schedule / demanding / not a good leader / defers to your mentor too much... here's how you handle that difficult post-doc in your lab." When you prepare to enter clinical years, you should seek out a clinical mentor as well.
The point of a rotation is not to get any science done. It's to determine fit, for you and for the members of the lab. Honestly, you can probably do that in most labs in 2 weeks. You are essentially dating the lab for a 4+ year relationship that is just as painful to terminate as if you were breaking up with a live-in significant other. Forgo-ing the M1 rotation will not set you to graduate behind unless you choose to do more than the required number of rotations (and see below why that will probably delay you less than settling on a bad lab). You will not be accomplishing anything meaningful during the rotation. In fact, you probably will be working on something completely unrelated to your thesis project.
If during the first week of the rotation, a post-doc tells you that you shouldn't come there, leave the rotation that week and arrange a shorter rotation elsewhere; don't waste your time sticking around for the full rotation. You don't keep dating someone once you realize it's not going anywhere. If several people tell you, "That lab is terrible for XYZ reason," listen to them. Don't delude yourself into thinking that your experience will be different. It won't be. The worst thing that can happen is you finish your PhD and hate your mentor and/or science, which happens a lot. Well, you could be kicked out with no notice and forced to restart or abandon your PhD... that would be worse (and it happens to about 5-10% of MD/PhD students in my experience).
If your program only gives you two rotations, and you're not happy after two, do more despite what they say. They may fight you, but there's really nothing they can do to stop you. I know far too many people who settled for "good enough" only to drop out or get kicked out after several years and forced to restart their entire PhDs and rotations. Honestly, the MD/PhD and medical school will tell you lots of things you "cannot" do but cannot stop you from doing it. Remember, they're in it for their best interest, and not yours, so do what you need to do. What are they going to do, write you a bad cover letter for your residency application? That certainly isn't in their best interest...
If you're interested in a lab, contact them NOW. Popular labs fill up and having waiting lists. Some people have to reserve their spot up to a year in advance.
