Basic strategy says at post. I don't do a lot of this because I am usually too busy to pay attention, but my new favorite example is the Super Bowl halftime. It opened Pit 1' and went to 3 at most places, and the total I think moved a point or so as well. Certain games are pretty easy to predict line movement on, so blindly you can go one way at open and then the other way at post. The movement tends to be in one direction only, although in the Super Bowl example, the line and total came back at the end, ostensibly people coming over the top at post.
I would expect the buyback to begin 5 minutes before post for halftimes or maybe an hour for whole games, so I would do my back right before these times.
