It depends. When DD is having a birthday party and there's 30 children, it's way too hard to open the presents as the children arrive and would take so long it would be rude. So then we wait until the guests have all gone home. If it's only a small gathering with a few people, then and there. But open cards first, to see the sentiment expressed and thank the giver before ripping off the paper to see the goodies!

I think the same goes for adult get togethers too, it depends on the number of ppl and how long they'd have to sit there and watch you if there's other things to do. I think the exception is a bridal shower or baby shower though, when everyone wants to goo and gaa over all the cute things.

I might also be sensitive if it's an event where not everyone has brought a present.