Whenever I go to an interview I try to collect the business cards of everybody I have any contact with during the process and they all get a Thank You email, in additional to the generic admissions office email. Most don't reply, but many do send very nice replies so I'm sure they really appreciate it. It may not help any but they do put a lot of time and effort in to the process and probably don't get much recognition for it. Plus, it's just a nice, professional thing to do. Then again, if they are on the fence about you, it could be an extra show of professionalism and enthusiasum that could seal the deal.