I think if you know who all contributed, it is always appropriate to write an individual thank you. If you don't know who chipped in (say the card says something like "The staff at X&Y Elementary") then a group note is totally acceptable.
I tend to go in on big gifts with my siblings and mom for baby and bridal showers and it bugs me to no end when the only person who gets a thank you is my Mom because she is the one who signed the card for us. My money went in there too.