+/- 10% is going to be fine (and remain unnoticed). Any more than this and it may become an issue as writing succinctly is a skill that is useful in business and business school. You should also realize your readers are reading multiple essays from multiple applicants so writing long essays is not likely to be a positive.

The exceptions to the above may be if you have a more unusual background (by business school standards) and feel you have a little more to explain (about your less traditional goals, experience etc.) In this case you may need to explain the nature of your experiences in more detail, but this does not give you the license to write at will!

I don't think there is a case where it makes sense to write much less than the suggested word count as the essays are going to be an important component to your application and should be used as such.