There's some truth to this, but I still think it's better to send one. Courtesy aside, it's just too hard to know which schools look for thank you notes and which don't care (and, within a given school, which interviewers like to hear from people they interviewed and which don't care). Of course a thank you note is nowhere near as important as, say, how well you actually did in the interview, but it gives you a chance to cast the interview in a positive light. Plus, having already written their critique by the time the note arrives, most interviewers will just take a quick glance and then forward it on for your file, where it will serve as evidence of your love for X school.
But yes, do be sure to use spell-checker.