For me it's:
1) If it is a pre-req and you received a C- or lower.
2) If you are applying DO and wish to take advantage of its grade replacement policy.
3) If it's required for graduation and you didn't do sufficiently well enough in that course to meet graduation requirements.
Those are the only situations where I personally would retake classes. If it's just another ordinary class AND YOU'RE PLANNING ON APPLYING ONLY MD, then just move on. Even if you got a bad grade in a non prereq, retaking it is the same as taking another class. The bad grade will still count in your GPA. I would just move on instead of dwelling on it.