Use your discretion based on the full details in your transcript. The issue is more about the story than the specific numbers; a drop of that nature might be worth explaining (depending on the real reason for the drop).
Here is Wharton's guidance re: the optional essay:
"The optional essay is truly optional, and may be presented in one paragraph of 250 words or less. It should be used to highlight any inconsistencies your application may need to address, or you can use it to explain your choice of recommenders, if you believe they are not a traditional choice. We recommend that you don't skip over any issue that may present a concern to an evaluator.
If there is a potential issue (e.g., one poor semester/term at university within an otherwise impressive academic experience, a year out of your career to explore), explain its context, what you learned from the experience, as well as how it may affect your time at Wharton. If you do not explain the context, the Admissions Committee may make uninformed assumptions." (http://www.wharton.upenn.edu/mba/admissions/preparing-successful-application.cfm#essay)