Emily Post Says:
Greeting Cards Need No Reply
Emily Post, the etiquette authority on, well,
most EVERYTHING, had advice for greeting cards, too!
This 1950 newspaper article indicates a change in previous etiquette
"Dear Mrs. Post:
Lately some of our friends have ignored birthday greetings we've sent them. Their reason is that it is no longer necessary to acknowledge cards of any kind.
I personally would think that lack of acknowledgement makes such greetings seem unimportant. What is your opinion?
Answer: The only acknowledgement that is expected of a greeting card is to send return cards at suitable times. To have to write a thank you for every greeting card would dampen the joy of receiving many!"
source: The Miami News December 28, 1950