Here in SoCal I seem to see the following trends:
Parties at non home venues (even at a casual venue like the park), never opened at a party
Most parties at a home and a party for <than 5 yr. olds, not opened
Larger parties of >than 10 kids invited at a home, not opened
Family parties at home, opened
Smaller kid parties with <than 10 kids invited, esp. when the kids are older than 5, opened
Gifts opened at the end of the party after the bulk of the friends have left, only one or two kids remain
We have done all of the above at one time or another, and each situation is unique. While I do like to see gifts open at a party especially for 1 yr. olds or kids over the age of say 4 so they can tell their friends "thank you" in person I am not offended if a gift is not opened in our presence. I don't see one wrong or right answer IMHO.
Dd1 is having a sleepover (yesterday to today) and invited 3 friends, they opened gifts after dinner last night.
I voted other