E,
I guess I'm old school. My kids have always sent thank yous and they are 23 and 24. When I was a child I always had to thank people for my gifts. I was taught to do that and I hope my children teach their kids to do that. I think its disrespectful to send a wedding gift or a baby gift and not be acknowledged. I would always think twice about sending future gifts if they can't take 5 minutes to say thank you.
I do agree with Dandy you send the gift because you want to, but if no one acknowledges the gift, you have no idea if they are even getting the money. You could always send a check and not sign it.... haha... I am sure you will get a thank you and the check back to sign.
I feel if you don't acknowledge a gift, it gets old, year after year. I would stop sending them. Sorry, just my opinion.