Extremely. If you don't waive your rights, (1) it will be assumed that you got extra LORs and discarded the ones that weren't so favorable, and/or (2) it will be taken for granted that this is not a candid assessment of you, since writers are expected to be more candid if they know you will never see what they wrote. So it basically results in them presuming that the actual assessment of you is something worse than whatever was submitted. Taking the "I have nothing to hide and I implicitly trust my attendings to only say good things about me" approach goes a long way.
As a caveat, even though I have opted for "Waive My Rights," half of my letter writers let me see my letter and "add anything" (aka nothing, because they were so good) they missed. This was true only of people I knew very well (working with for a year or more).