We don't do cards per se. If we receive any kind of gift via mail, even if it's a birthday card, a handwritten note on nice stationary, thanking the sender for their thoughtfulness is always sent in return. We do live a long ways away from family and loved ones, the ones who send birthday and christmas gifts and I know that the folks back east or down in Arizona, or even Grandma in Nampa, appreciate it when especially my kids send a handwritten thank-you note in response to a gift. It's only good manners.
If the giver is present at the time the gift is recieved, then the personal thanks are always given with a big hug. I really don't see the need to send along a thank-you note if the giver was thanked personally. The important thing is that any gift or sign of thoughtfulness be gratefully acknowledged. The only exception I can think of is Christmas cards. I truly don't expect thanks for a Christmas card nor do I send a thank you note for one.
I guess a good rule of thumb is to return thanks the way the gift is sent. If the gift is given in person, thanks must be given right then and there. If a gift is sent via mail, then a nice note should be mailed in return as soon as one can grab the stationary and pen.
What I don't appreciate is what appears to be a current trend: The mass generic "Thank You" for wedding gifts. A cousin recently married. I was a little taken aback when, inside the invitation, we also recieved a card stating that the couple would appreciate only monetary gifts that they could spend towards their honeymoon. As this was at the time that the storm blew the roof off of the house, we had no extra cash. I was struggling hard enough to finance my groceries, much less my cousin's honeymoon, so I wound up not sending them a gift at all. But, even though we didn't send any cash, (which I was planning to as soon as we could afford to) we recieved a printed generic Thank You card with their names printed, not signed, on it. That's all it was too, a small card with Thank You printed on the cover, "We appreciate your thoughtfulness" and their names printed inside. Nothing more. Frankly, I sort of washed my hands of the whole tacky mess and haven't sent along any gift. Their combined incomes far exceed what Steve brings home, and I know that they already went on their honeymoon. As I wouldn't want to clutter their house with unwanted junk, which apparently they would think a gift that didn't include cash would be, then I'm sure they're just as happy.