Always choose to waive it.
First, if you don't, no program who is reviewing the letter will trust whats in it. Programs absolutely see whether or not the student waived their right to see it.
Second, I wouldn't waive the right then ask the letter writer to see it. While I agree, technically, this is probably ok from a legal standpoint, it also may make you appear dishonest to the letter writer, the very person you just spent a month trying to impress to give you a LOR. The last thing you want to do is shoot yourself in the foot right at the end. I can tell you if any student ever waived their right to see a SLOE I wrote for EM, then asked to see the SLOE, I'd immediately find them dishonest in my book and I'd even consider mentioning in the LOR itself.
Part of actually evaluating and stratifying students, rather than giving everyone the same grade, is actually evaluating them and doing so without them looking over your shoulder. I think in EM we nail this perfectly with our SLOE process. I wish other fields did the same. Students have to accept this; if every single student all had perfect LORs that all said they'll be the perfect (EM/IM/FP/etc) doc, then all the LORs become meaningless. There needs to be an honest assessment that stratifies student performance. Just accept that fact, work hard, and hope that your "honest assessment" reflects you being a good student, assuming you were a good student. Trying to control it by manipulating the system will just lead to you appear dishonest I'm afraid.
If you were 100% freaked about by having no clue what your LORs say, perhaps your schools dean will recommend what LORs you should send to programs? This isn't actually you seeing the LORs but they could skim them and give you an idea if there is a LOR to absolutely NOT include.
