I got a C- in Orgo II and did not retake it. I had extenuating circumstances that semester - a very traumatic issue to deal with. I wrote my AMCAS essay about the issue and "subtly" mentioned that it caused my grades to drop that semester. On interviews, I emphasized that my other grades are more reflective of my academic ability. I am sure it hurt me with a bunch of schools that rejected me pre-interview, but I got into my #1 choice.
If you can make a good argument that your C- is an anomaly, and that there is a good reason that you bombed the class (ie death or illness in the family, not just slacking off), then you should be ok. It can't hurt to retake it though.
Good luck!