The statement that "you MUST waive the right to read the letters [referrees] put together for you" is inaccurate. It has always been the case that confidential letters are given greater weight than private letters, and thus waiving the right to review is a good idea. However, it is a choice you must make, which is why that checkbox to waive or not waive is included in the cover sheet for references.

Regardless, the career office dossiers will include the same waiver of your right to review the documents, so the same benefit is gained by having a dossier set up there. It just eases the work of sending & compiling for you and your referees.

I would also suggest that you ask referrees for copies of the recommendations regardless of waiver. After all, folks who are writing for you generally are supposed to like you, and as such are saying nice things about your candidacy. It would be helpful for you as an applicant to have a full picture of what is being painted on your behalf to admissions committees, and most folks who like you would be happy to give you a copy of such nice sentiments. There are, to be sure, sticklers who would object, which is fine, but I think you'll find most amenable to the idea of crafting a good holistic picture of your application for the greatest opportunity for success.