Depends on what you want to do, and where you go. For me, I do not want to do anything competitive (EM Residency), and I go to a H/P/F school with no class rank.
So, Pre-clinical grades do not matter, Step 1 score matters, Clinical Grades matter, and Letters of Recommendation matter.
The best thing you can do is get more of an idea of what you want to do then work as hard as you need to achieve that goal.