To be honest, its the way I was raised, we NEVER opened gifts at the party. For a two reasons.

1. Alot of kids want to play with everything thats opened, including the birthday kid, and it detracts from the party to have 20 kids screaming to play with the opened presents. And they all do it up to a certain age. I also think it distracts the birthday child from the party as well, I know if I opened a kick butt present Id want to play with it then and there too. Its rude to ignore guests.

2. It can make people feel uncomfortable because some party guests may have spent more than others on said party gift. Some people may not have the money to get something fairly expensive, while others might have the financial means or the personal reasons (ie immediate family, godparents, close friends, whatever) to want to buy something bigger or more expensive.

We've been to parties where presents were opened at a specific time during the party or as they arrived; and while it doesnt matter to me how someone else runs their party, it does make me uncomfortable and most of the time causes issues amongst the kids (generally the younger ones ie under 10).