At my school, most people that are "ahead of the game" take orgo and physics together sophmore year. This means that they are done with all pre-reqs before taking the MCAT. Most people applying to med school usually are completing one of the pre-reqs while studying/taking the MCAT in April their junior year. This being said, I would hold off on taking it untill you survive the gen chem/gen bio at your school your freshman year, these can be very difficult, sometimes more so than orgo, genetics, ect. I would fear bad grades (at least if I had done this in my case) taking genchem1, gen bio1, and physics 1. This is usually because you are so unprepared and don't know how to study yet (or want to party all the time hehe). I wouldn't have changed the way I did my schooling and will only have to take like 20 hours my senior year to graduate, plus I can setup a REALLY easy semester next April so I can focus on MCAT. Here is what I did for my pre-reqs:
Freshman year:
First semester: Gen chem 1, gen bio1 (usually molecular)
Second semester: Genchem 2, gen bio 2 (usually organismal)
Sophmore year:
First semester: Orgo 1, Physics 1, genetics
Second semester: Orgo 2, Physics 2
Of course along with pre-reqs take your required courses to complete your major. I also took a lot of summer school and came into college with like 24 credits so I have some flexibility to take like 12 credits a semester and smoke my classes. I think that this is more important than taking like 18 credits and doing OK and graduating early which is what some people do.
Hope this helps.