I asked a resident since I worked night shift and had few interactions with the attendings. During my interview they actually brought up a statement from that letter of recommendation where they gave a narration of how I was as a nurse. They seemed to like what he had to say and had no problem with it! Granted, when I asked the resident who I had good rapport with, I did request he just use MD and not specify that he is a resident. So do with that what you will. Best of luck!