As stated above, the standard is that, if at all possible, you want to waive your right to see the letter, which means that the letter writer has to send it in without your seeing it. If you send it in, it means you had access to the letter... some programs think that letters that the applicant has seen are worth less because the writer may have felt pressured to leave out anything bad. I don't know how important this is, but most US applicants do not ever view their letters. There were a few people posting that did not waive their right to see the letters that had different arguments, so you might want to search about this waiver.
Bottom line is that if you can give the letter writers envelopes with postage pre-paid, along with the cover sheet/waiver, this is the best route.