I was in the first batch of interviews. So far no one has confirmed a letter of acceptance. The webtool has said provisional acceptance for some applicants. However, UCSF did mention that the letter would be the official acceptance because of the rolling admissions process. It's going to be a long wait!
My web status states that I am missing LOR's. But I already brought that up tp Barbra and even saw them in my file. So there is your answer. NOt too often.
I would wait for the letter.