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.
My opinion is the same. However I do find it quite awkward at baby showers if someone has spent quite a lot on a gift and then someone else has spent a minimal amount. Dont get me wrong I am not a shallow person and I believe its the thought of giving not the cost, but I cant help but feel awkward for the people that purchased the gift if something massive gets opened by one person and then the next gives a bib etc.