From whatever i've heard its a good idea to send them. I sent it to both the places i interviewed at. (they were both thank u emails and i got accepted to both those schools, prolly not on that basis tho). In addition the sooner you send them, the better it is. Although i'm not sure if somebody actually looks at them or not, but from what i've gathered by talking to adcoms at some schools, they keep track of all correspondence u have with them and if nothing else it shows you are still interested in the school.
And as somebody else said earlier, it really doesn't hurt to send it.