I generally assume (undergraduate) students know to waive their rights to look at a letter of recommendation when they ask me to write one, but I recently ran into a student that did not. I explained that my default was that they had to waive their rights, and I'd send in the letter after the checkbox had been changed. If you don't look, though, I find it's easy to miss which box they've checked as you're filling the form out on line.
I simply prefer my confidential letters to be confidential. I understand that waiving their rights is not any sort of guarantee that they won't see the letter, but I think the understanding is important: I'm writing an evaluation of them to my colleagues, and that information is, as a matter of default policy, not meant for their eyes, so I can be forthright in expressing my opinions.