This one is an absolute. There is nothing more important about your LOR than making sure that you waive your right to see it. If you do not waive, then PD will know that you didn't waive the right, and will become suspicious about your reasons. It inherently means that you don't trust the physician to write you a good letter sight-unseen, and that the letter isn't a true representation of your abilities.
So make sure that your writer can write you an excellent letter: (Dr. X, would you be willing to write me and "outstanding letter of recommendation" ) In most cases if they can't make it outstanding/excellent then they will decline and you should search elsewhere for a better letter.
Also, if you find an excellent writer, they may send you a copy on the sly (a violation), or if you interview at a certain few programs (which will remain nameless) you will probably get to see the letter anyway because they include it in your packet of materials on the interview day.
