<p>Waiving the right to see your letter doesn’t imply that your recommender is not allowed to show the letter to you before sending it, merely that you don’t have the right to request to see it after you’ve enrolled at the institution.</p>
<p>there are just way too many professors in this world, I believe there is no “guidelines/instructions to write LoR for your students”. So just go with the specific culture!</p>