Hopefully you will get advice from people with more recent knowledge, but I figured I'd chime in anyway.
I prepared for the MCAT way back in the day (1991) by working through the Flowers prep book. With regard to timing, a thing to think about is developing intellectual maturity in the subjects being tested. This allows the student to think about the questions with more flexibility than if they just learned the material. So if it were me, I'd take it as late as possible. This is assuming that what is tested is in line with the student's major so they've been working with chemistry, biology, etc the entire time.
