Ideally, you'd want to take it at the end of your 3rd year (spring of Junior year). However, many people take it in late summer right at the beginning of 4th year, which is what I did. It did put me at a disadvantage with a few schools, as they would not schedule an interview until they had my MCAT scores. As a result, some schools had pretty much already filled their classes before I even interviewed.

Aside from that, there is no real difference between when you take it. I would suggest taking it as soon after you've finished your pre-med courses as possible, while the material is fresh in your memory.