Most books say to waive your right, it allows your recommendors to write candidly. Apparently some admissions people feel that you should waive your right, some don't really think it matters. My opinion is that you should do it. You will only see the letters after you get accepted, so it's not like it will affect which letters you send out. Just ask people who will write something good.
Also, your professors might just let you look at the letters anyway. One of mine is sending me a copy of what she sent to lsac so that I can review it, and another one basically told me what was in the letter.