In my opinion, you should absolutly always waive your right to view the letter of recommendation.
Also, there is a small caveat here. Even if you do not waive your right, most schools will only allow you to view the letters AFTER you gain acceptance. If you do not get in, they can deny you from seeing the letters even if you did not waive your right.
I think not waiving your right would raise some eyebrows. Even so, its a very minor thing I would imagine.