Depend on the party.
Eg my daughter third party was small, so opened as arrived.
Baby shower - at set time, as people seem to want to all look at what you got.
Engagement /wedding/21st type- after guests have left.
Pretty much exactly the same

Problem we had with opening DD's at her party is that my niece wanted to play with them all, which is ok except that some of them my DD was quite attached to and didn't want to share and niece kept chasing her round saying "Share! It's not nice to keep it all to yourself!" and I was there going "Niece, leave her be! They are her special presents!"