Basic science year grades aren't particularly influencial in terms of matching. But it's harder to come from being a C student to the dude who rocks Step 1, and Step 1 has a huge impact on matching. So you are really laying the ground work (or not) for that test moreso than your first year grades are going to matter. Also the clinical year grades are going to to some extent turn on how much background stuff you can mine out of your memory during pimping. So it pays to push yourself.
As for C=MD, it's true for the US allo crowd. There are still more residency slots than US seniors, so if you set your signts correctly, you will land someplace. However most people in med school don't just want to be an MD, they want to be in their dream specialty. If your dream specialty is psych or peds or FP, you are good. If not, you may need to work harder. A few C's, particularly in the first year, won't have that much impact. Keep it up and you may be causing yourself some angst.