It would seem to me that not waiving your rights to LORs would lessen their impact. An admin committee might see that your reference felt obligated to write something nice simply because you would be able to read what they chose to write. If you waived your rights, the adcom knows the reference would have been able to be completely honest in their opinions, knowing that you would never read what they wrote. Make sense? One of my WONDERFUL references, a pharmacist that I work with, allowed me to read her recommendation even though I waived my rights to do so. That is an awesome person!
I would always, always suggest waiving that right. This means you will have to put a lot of thought and trust in each reference.