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.
Yep, ditto, although I do prefer to open as they arrive. With my engagement party there were just far too many people all arriving at the same time to open as I went, so the plan was to open them after. But then my mum dragged me inside half way through the part and said "so and so wants to see you open their gift", word got out that we were doing presents, and I had to open all of them with everyone crowded into our lounge room watching. Embarrassing!

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