I recieved A's in Bio and did very poorly in physics C- both semesters. I also got a C's in organic chemistry. In chemistry I got a B and a C+. Either way I don't really remember much of any of the subjects because the last time I took them was during my freshman and soph. years (back in '02 and '03).
From my experience, most of the students that I know who enrolled in post-bacc programs did so b/c either they haven't taken the required pre-med classes, or they did take the required pre-med classes but it was long ago and they performed poorly in them.
Therefore, the answer to your question depends on what your goals are; do you want to retake the classes because you want to raise your GPA or do you want to retake the classes purely because you want to understand the basic sciences better so that you can improve your performance on the MCAT?
The other posters here are correct in pointing out that a college level courses in physics or general chemistry is not a great way to ready yourself for how those sciences are tested on the MCAT. If your only goal is to improve your MCAT score, then perhaps enrolling in a post-bacc program is not the wisest idea.
That said, based on your comments, it seems that a post-bacc program would help. One, you indicated that you did not put up great marks in those college classes. Two, you indicated that you have already prepared for and struggled on the MCAT. Perhaps the root of your struggles is that you failed to develop a strong understanding of the material when you first encountered it. To varying degrees, most retail MCAT preparatory materials assume that you used to have a good understanding of the material and that all you need is a thorough review. However, if you never received a solid grounding in physics in the first place, then a thorough review from an MCAT prep book may not suffice. Ultimately, you know you best so take some time to write out the pros and cons of each option and I think by the end of that exercise it will become clearer to you which path to choose.