Some admissions officers report that the first batch of applications tend to be from candidates with strong qualifications, confident of acceptance. In this case, you might not be the very first one on line; but closer to the front is still better than lost in the heap of last-minute hopefuls.
Of course, if applications are down that year at all b-schools or -- thanks to the latest drop in its ranking -- at the one to which you are applying, then filing later means you can benefit from admissions officers desperately filling spaces. But this is risky business, especially since the rankings don't come out until the spring.
Conversely, if the school to which you are applying was recently ranked number one or two, applying early may make only a marginal difference. Swings in the rankings from year to year send school applications soaring and sagging. From beginning to end, a newly minted #1 or #2 school will be flooded with applications.