Overlearn in undergraduate College - good or bad idea?
Having a good GPA is important; having a great GPA is not.  Assuming your GPA stays above a certain threshold (about 3.5), other factors like research experience are significantly more important, at least for admission to the best departments.

In particular, if you get a C in a distribution class because you spent "too much" time on that experiment you later published, nobody will even notice the C on your transcript.