Most of the rules sets forbid you asking. If asked in that way to me, I would be offended and refuse the letter even if positive. That said, it is considered part of the unofficial rules that if you are going to write a negative one for an applicant, you should offer them that "I probably am not the best person to write this recommendation." Extremely negative letters can be followed up on, and most letter writers who are experienced will not accept an LoR request if fundamentally negative unless compelled.Hi! My research PI is writing me a letter of rec. While I believe I was a good research assistant overall, she sometimes made it clear that she wanted me in lab more often than I was able to be there. Would it be disrespectful to send the following message or something similar after she submits my letter?
"Hi Dr. __! Thank you so much for submitting my letter of recommendation, I really appreciate it! I hope this is not rude of me to ask, but I know that I may not have come into lab as much as you wanted me to at times, so I was wondering if you could tell me if your letter contained any negative evaluation of me? I respect your opinion of me whether positive or negative, and I would never ask to read your letter, but I am hoping you would let me know if there is anything written that I should be concerned about."
I've obtained more letters than I need to send to schools, so if she wrote something unflattering then I would choose to send a different letter. Thank you!!
In practice, what I do is to have the LoR recipient read my letter and check for typos (even over my screen if AMCAS Interfolio), negotiate edits, and then I submit IN FRONT OF THEM to ensure that we are clear. In the cases where I write on University or Government letterhead, electronically sign using an e-signature, print and wet sign, seal the envelope in front of them and sign over the seal and hand it to them to mail and they write me a handwritten receipt that I keep on file for two academic years (I refuse to take responsibility for lost mail). For the future though, do not work for someone if an LoR is an expectation and you cannot trust them to do it right. It is a bad character read, and that is one of the hidden qualifications for this.