If you had a lower GPA, it would help you to get straight As in upper-level science courses. But your GPA is fine, so there's no point to taking extra classes. If you want an advantage in med school, you could learn the material on your own, or pay big bucks to audit a classroom or on-line course (no point in lowering your GPA if your work will keep you from putting your full effort into the course).
I recently graduated (2008) and I am currently working full-time and planning to apply to med school in 2009. I was a psych major in college but took the basic premed requirements (year of biology, year of chemistry, year of orgo and a semester of Genetics). But since I only took the minimum requirements, I was thinking of going back to school this spring to take more science classes before I applied (Biochem, Anatomy & Physiology and another upper-div bio class). My main concern is that if I went back to school, I would have to quit my current job. So I was wondering, would there be any advantage for me to back to school to take more classes? I graduated college with a 3.7 GPA and a 3.6 science GPA.