Well, we've always had an artificial tree (I was allergic when I was a kid and we never really re-evaluated) so the timing isn't necessarily important. We usually wait until everyone is home, then we sort the tiers and put it all together. We put up the lights (alternating color and white) and tinsel, then dad disappears and us ladies put on all the ornaments.
Actually, we just got a new tree! Our old one was nine feet tall and too big for the new house. So it's going to be quite different this year--it's pre-lit, and so I assume it stays assembled. It's probably already up, but we'll wait until Nora and I are home for Christmas to decorate.
And, behold! This is my final photo project--I printed pictures of foliage in crazy, whacked out colors. I'm so proud of it!