when do you let them open presents?

[2CheekyMonkeys]

just wondering the etiquitte of opening presents at birthday parties. do you let them open the presents as guests arrive and hand them to the child, do you collect all presents on a table and open them all in one go at beginning of party, or at the end of the party or when guests have left? or does it not really matter? it would be for a 3 year old party. :-)