I retook both Gen Chem I and Bio Sci I. However, I'm a non-traditional student and retook the courses b/c my grades (and knowledge) in them were almost 10 years old when I applied. No one ever mentioned anything about them in any of my interviews, but I had not failed the classes previously. I don't think retaking courses is a big deal, but I wouldn't do it unless you failed the class or it's been a long time since you took it.
Just my $.02.