n the business of recruiting I've found the smile to be a powerful bridge-builder for friendly and even candid conversations. So I always use it and welcome it from candidates I interview. It is the universal body language of: "Welcome" and "How are you?"...and, "I hope well..." And, "I am actually listening to you because I care about what we're discussing."
Use it or lose it--the SMILE, as an important piece of the human condition, particularly in the realm of communications.
Going the other way, with the more formal "poker face" that essentially gives no signal of warmth, or even that I'm listening--I feel will work against an interviewer. It'll make the interviewee ill-at-ease in the job interview setting. And unfortunately some interviewers want the exact condition in the job interview.