Let's ask the residency program directors:
http://www.nrmp.org/data/programresultsbyspecialty.pdf
The short answer is that everything counts.
Long answer: at my school, your academic performance over the first two years is summarized in a paragraph on the dean's letter. If you have done exceptionally well, it will read well. If you have done less than well, it still won't sound so bad. The programs will see that you have honored, high passed or passed your classes. That said, much more attention will be devoted to your third year clerkship grades and even to your narrative assessments. While some people do very well for themselves in the match having performed less than stellar in their preclinical courses, there are a number of reasons to work hard in those first, couple of years.
- You might discover a hidden interest or talent in one of the sciences (histology, anatomy, pharmacology, etc) that leads you down the path to an unsuspected career decision.
- You will have an easier time preparing for Step I when the time comes.
- You will have an easier time understanding some of clinical medicine, although I agree that some of what is taught in the preclinical curriculum stays in the preclinical curriculum.
- You help your grade point average, class rank, and chance of being elected to an honor society. The stronger the reputation of your medical school, the less this will matter provided that you are smart, hard working, and interview well.
- You will feel happier and more confident with yourself when you learn the material and perform well on exams.
And there are a number of reasons to not-kill-yourself in the first couple of years.
- Your third year grades at many institutions are weighed much more heavily than your preclinical grades in determining your class rank.
- Your grades in individual classes are less important than your Step I score (in the sense of getting interviews and matching).
- You could use the extra time to have a life or pursue research interests. Research and research-like activities will help you make valuable career decisions and let the residency selection committees know that you are serious about your chosen field. If you do not care for research, then you can take time to explore the fields that interest you.
- P=MD, 7O=DO... depending on where you want to live after medical school and into which specialty you would like to match, you may not have to look like a future Nobel Laureate on your residency application.
- Your friends, family, classmates, body, mood, and (if you believe) soul will thank you for not being a gunner.
Do what you decide. The only piece of advice I'm qualified to provide a first year is that you should keep an open mind.