You open presents or not, its up to you.
Opening presents at a party, can take a LONG time. Then photos are taken as the child opens them, then the parent has to write down who gave what for thank you cards as you open them, then you ooh and aah at the presents, then thank the person there.
It can literally take, over 1 hour. Just to open presents. I have seen this happen.
Meanwhile the kids get antsy and will get distracted and play or fuss.
I have seen this happen.
For us, we open presents at home. After the party.
Kids are tired at parties, if it is a long party. Too.
If you open presents at the party, you HAVE to, incorporate this timing, into the party's timeline... and do it so that the party ends, when the invitation says it will.
People/families, plan their day, by when a party starts and finishes.
Especially if their kids naps or they have other things to do.
Unless, it is an ALL day party.