If you indicate on the ERAS form that the letter is "closed", your writer will likely write that on your letter, something to the effect of "Mr. X has waived his right to view his letter under the Family Rights and Privacy Act" or something like that either at the beginning or the end of the letter. In fact, the instructions on the ERAS form indicate that the letter writer SHOULD write this on his letter. This does give a letter more credibility -- put yourself in the shoes of the interviewer. However, if you've NOT waived your right to see your letter (open), I'm not sure whether they indicate this on their letter. All of mine were closed so I have no idea.
Having said that, I'm not sure how much time interviewers spend dissecting letters to see if they were open or closed. At many of my interviewers, they just skimmed through my letters and looked at who wrote the letter more than anything. But other places spend more time reading the letters.
Bottom line is that if a letter writer likes you then there's no way he would screw you over and write about your weaknesses...this is especially true if all your letters are from orthopods because they know the game and how competitive it is. But if you asked a medicine attending (those anal jerks), you may find them writing about your weaknesses or anything less than flattering.
I asked an attending at one of my interviews (at a dinner type social) about the letters and how difficult it must be for attendings to write so many letters, etc. He basically told me that if they like you as a person, they'll try to help you out by saying nice things about you. They don't have to know you for 8 years or you don't need to have published 5 articles in JBJS for them to be flattering in their letter. If they like you in the brief or not-so-brief interactions with you, AND AGREE TO WRITE YOU A LETTER, then you can be pretty sure that the letter will be pretty darn good and that they would never list any weaknesses you may have.
Also, and this may be most important factor of all -- put yourself in the shoes of your letter writer. If you do not waive your right to see the letter, how much confidence are you placing in this person's willingness to write you a good letter. If I were an attending and genuinely liked a student, but if that student did not place any confidence in my willingness to write him a good letter, it would rub me the wrong way. By indicating that you waive the right to see the letter, that indirectly communicates to the attending that you have confidence that they WILL write you a great letter and that you WILL send their letter to the programs you apply to. Think about it, is he likely to spend more or less time on a letter that he's not sure will even be sent to programs??
I'd say play by the "rules" and waive your right to see the letter. Not only from the standpoint of the interviewers, but more importantly out of respect for the person you ask to write a letter for you. It worked for me and most people I spoke to who went through this. If you got the "goods" (grades, scores), don't worry about it. They'll write great things about you (unless you're a social idiot).
I hope this helps.
(By the way, the reason I know what my letters said is because at some of your interviews they give you your file for you to carry them into the different rooms -- everyone just peeks at their letters!)