From what I've seen, I would advise against taking it your sophomore summer. A lot of my friends did that and ended up regretting it. Furthermore, a lot of them did poorly in their prereqs because they had to take so many at once. It's kind of difficult to deal with a bunch of science-intensive classes at once (at my school anyway). Since they did poorly in those classes, they also didn't do very well on the MCAT.
I would suggest spreading out your prereqs and substitute some liberal arts courses or something to show that you are making an attempt at being well-rounded. It will also give you a chance to take upper level science classes and become a better critical thinker--something the MCAT tests you on heavily. Intro courses are basically giving you the knowledge/facts. Critical thinking comes later. Overall, there's no rush. If you prepare yourself, you can take it early in the summer of your junior year and get applications in just fine.
That's my two cents from my experiences. Good luck to you.