I polled my kids...Both sons agree a thank you note is a must. One DIL agrees..other one thinks an e-mail or a text msg will do...Guess you know which one I prefer!
My children were taught to write notes when they were very young..Christmas morning gifts were opened and right after dinner notes were written. They got "Thank You" notes in their stockings. They still write them especially when gifts are received by mail. I believe it is simply good manners and I love the practice.