Any that I have wrapped and ready for extended family and friends go under the tree as I get them wrapped, maybe 2 weeks before christmas, and any that arrive in the post for the kids go under as they arrive (we have family interstate).
A few days before Christmas I will put a wrapped present for each of the kids from us under the tree, just to add to the excitement.
Christmas Eve the kids put their sacks under the tree. Santa fills their sack and leaves one wrapped present on the end of their bed - this way they have something to open if they wake up early, they are allowed to get up and wake DH & I from 6 o'clock onwards and then we do their sacks.
Presents to them from us that are under the tree get opened after we have eaten breakfast. Then after lunch we open presents from grandparents and cousins etc. I love spreading it out a bit so it's not a mad rush of paper.
Also on Christmas Eve, after dinner, we open presents from people who are not with us on Christmas Day. This has become a really nice tradition and means I can see what they are getting from the in-laws and a couple of overseas friends who send them stuff.