If you take Quiz 6 before the end of exam period (i.e., by May 5th 4:00PM), then I will discard the lowest amongst your six quizzes, and use the five remaining to compute your final grade.

If you do not take Quiz 6, I will use your five quiz grades to compute your final grade.

Taking the optional 6th quiz therefore cannot lower your grade, and will improve it if you score higher on the 6th than your lowest score among the previous five.