uncjules -
If you are going to retake any class, you should be sure that you would make an A in the course. I haven't retaken any courses, but I transferred from Carolina to IUPUI and a class didn't transfer directly (I was in an honors program and they combined Chem I with Chem II into one semester), so I took Chem II again and earned a 4.0. An important note is that your grades will be averaged together if they are the same class, so if you make a 2.0 in the course the 1st time and make a 3.0 the 2nd time, you're reported grade on your application at the optometry school would be a 2.5.
As cpw said, a C isn't 'bad' in a pre-requesite course as long as you make an A in an upper-level course in the subject area. Freshman-year are important, but not as important as junior-year grades.
Nick