I think it would be tacky if you did NOT give a thank you card. You are asking a person who really does not know you that well, who is likely overworked, and who may have several letters to write, to take the time to write you a letter which you are hoping (even expecting) to be an incredibly complimentary testament to your character. This is definately worth the little time and money that it takes to give a personalized thank you note.
Just my opinion. But I also get annoyed if someone doesn't send me a thank you note for a wedding gift I gave them.