Technically no, but it is recommended. The letters may not hold as much weight if you have not waived your right. You should waive your right to give yourself the best chance to benefit from the letter. If you know the letter writer well enough, you can generally assume that he or she will not hurt you with the letter. You may want to consider asking your letter writer whether he/she would be willing to support your candidacy to medical school with a strong letter when you make the initial request.