I always have my daughter send thank you cards. I feel that it is necessary to teach her that she should be thankful that someone thought about her to give her a gift. So even though she does a verbal thank you, it is always best to send a card. It certainly can't hurt anything by sending an extra thank you and because she is only 5 she adds her personal touch to the cards, which makes them extra special.
Also, my husband owns a business and for customers who refer him to other clients he always sends a gift around Christmas--not cheap gifts either, usually a value of at least $100.00--in all of the gifts that he has sent we have never received a thank you--a card or a phone call. So, I always call to make sure that they have received the gift and then they say, "Yes, we have and thank you." It is a sad state when you have to call to make sure that they received the gift and then and only then will you receive a thank you.