No, it doesn't matter how you write a thank you note, but, as mentioned above, just don't write one at all.
Chances are they won't even remember you and no one (really, not one faculty member or fellow student interviewer) has ever said, "gosh, it would be really nice if these kids wrote thank you notes." While my classmates were very conscientious in writing evals, very few remembered their interview subjects once the evals were turned in.
I really wish I had listened to this advice when perusing SDN during application time because then I wouldn't have wasted my time writing nice thank you cards.