LOR's carry much more weight when the readers know that you don't know what's being said about you. You waiving your rights to read a letter allows your letter writer to be as honest as possible without fear of you actually not sending that letter out because of what they've written. That's why most applications pretty much require you to waive your rights to read your rec letters. On the other hand, if the professor decides to show you the letter of rec on his/her own volition then it's fine to read it to my understanding.
If you have a hunch that a professor has a reason to dislike you or is just a negative person overall, it's better to just ask someone else for a letter. I would even go as far to say that if you're not close enough to your rec writer to confidently gauge their perception of you, even if he/she did write a letter for you, it's guaranteed not to be a strong one. Find rec writers that you can tell are rooting for you.
That being said, I'm pretty sure AMCAS doesn't actually have waivers for your rec letters, but most pre-med committees or health professions offices for colleges will. Personally, my school had a waiver that we had to hand to everyone we were getting a letter from.
Good luck!