The best time to take the MCAT is immediately after you have taking the prerequisites for medical school.  Having taken 1 year physics, 1 year biology, 1 year general chemistry, 1 year organic chemistry, and 1/2 a year of biochemistry will give you the foundations for the MCAT. Don't wait too long before you take it.  If you do, you'll forget a lot of the information.  It's best to take the MCAT when you have everything still fresh in your minds.

Also, it is helpful to take an MCAT review course like Kaplan or Princeton Review the summer or spring right after your last premed class.  The MCAT course will be a review for you.  It shouldn't have to be in substitution of your classes if you've forgotten the material since it was a while since you took the class.