One thing to remember though is that while most MD schools don't have letter grades (A, B, C, F), many do have a tiered grading system (Honors, High Pass, Pass, Fail), which comes out to pretty much the same scale. Its not a straight P/F scale.
Ultimately though, as people are saying, preclinical grades tend to mean very little, provided you pass everything. Clinical grades tend to mean much more both in terms of class rank and in terms how programs evaluate you. Again, though, they are still low on the list for what programs look at.
Aim for doing your best, first to pass, then to retain/understand the material. Beyond that, don't kill yourself.
Unfortunately, my school limits your rotation options if you get below a 2.5, and if you get below a 3.0, you have to attend a board review course in the final semester and aren't allowed to participate in school clubs. So there are extra goals you need to set for yourself to not be "at risk".