Okay, apparently, I am the cad among all mamasourcers.
Personally, I think that, yes, etiquette dictates that they should be sent. And I agree that it is nice to receive the personalized handwritten thank you note...however, I just don't have the time (not to mention the mental capacity!) to do it every time.
I make sure to send out Personalized Thank You's for the "big" things like wedding, shower, etc. However, I am just as happy when I receive a phone call or better yet, a IN PERSON "hey thanks, I LOVE "x"". As long as it is acknowledged that I took the time, not to mention spending the money, to get "you" a gift, I don't think ill of the receiver if I don't get a "thank you" in the mail.
That said, I am attempting to teach my son to send thank you's...even a scribble/art on a piece of paper...but if someone thinks that I am not civilized because I didn't send a proper thank you card for every gift we've received...all I have to say is...oops, sorry.
So, personally, I would say as long as the gift/giver is thanked in SOME FASHION, you're good...ideally, send a note card...however, I would like to think that most busy moms would understand if they didn't receive a card every time.