I would say that you should stay within 5%, and stay under if possible. If the adcom notices that you went over, they will definitely look negatively on it. Think about it... what better way is there to show that you can't follow directions? That you think you are better than the rules? And that you can't write concisely.
Going over 5%, you run the risk of the Adcom noticing. They read 100s if not 1000s of essays, and can EASILY tell if the essay is over. They have seen every usage of margins, font size, etc.... and probably could guess an essay within 5% of length just by looking at it.
Why risk it? More is not always better. Some of the best writing is concise and to the point. They know that it is hard to stay within the word limits, but they expect it b/c it's part of the rules. If you can't follow directions or rules, why would you be a good business student?
If you go 5-10% over it probably won't be bad... but anything over 10% over could really be looked upon negatively, b/c it will be a blatent disregard of rules.