An away may not hurt you. At my program, I occasionally have the opportunity to evaluate students I work with. More than once I've called the PD up and said, "This guy's a stud. He's smart, has good hands, and knows when to stay out of my way. Once we got to PACU, I'd send him to see the next patient and start the IV. He nailed all the intubations and IVs. We should take him if we can get him." I haven't had to do the, "This guy's a prick. Don't waste an interview," but I know people who have, and I trust their judgement. I have done the, "This person is really nice and tries hard, but isn't in the same league as our residents. He'll be a good resident, but our program can do better." I know the PD listens when I or other residents call. That said, an away clearly can go either way. However it's not a necessity, unless you want to try a place you're unsure about or if you don't have a home program and need some exposure and a letter.
