Bishop Coyne answers these two questions:
I believe it is a piece of French Canadian folk wisdom, if you will, that if you show up before the offertory prayer, the Mass is valid for satisfying you Sunday obligation. That's the convention I have worked with (not that I show up that late for Mass.)
As for leaving, in my family, we leave after the priest leaves the Mass. I think of him as the "host" of the Mass, and it seems to me rude to leave before he leaves. Especially since he often has to make it down the aisle, and you don't want to get in his way.