I wrote update letters for schools whenever I had a chance (post-interview). I didn't make it annoying but I think I sent two update letters to all post interview schools, and three to a couple. Some of these were sent before decisions, and some were sent post-waitlist status. I agree with the previous poster that it is a good way to keep your name continuously on someone's mind. I am not sure who reads the letters at each school, but it can only help as long as they aren't too frequent. The more times you can get your name mentioned in a good way like this, the better right? Good Luck.