If your numbers are below the 80% range, they warrant an acknowledgement and an explanation. Similarly, if extenuating circumstances caused the low numbers, those circumstances are an important part of your profile. The optional essay is often the place to make these statements. If you write the optional essay, make it short and straightforward. Provide a brief explanation, take responsibility, and focus on evidence of your talents that counters the impression made by the low stats. Also, explain (or, ideally, show through example and anecdote) that either you have dealt with the problem causing the poor grades, or the circumstances no longer apply.