Best rules to follow betting overnights IMO, with the obvious implication if you are going against any of these do the opposite:
1. If your team is "hot", bet them overnight.
2. If you are betting a sizable dog, bet it just before the game.
3. If you are betting a "name" pitcher, take it overnight.
4. If you are betting a big name team, say Yanks, with a bottom of the rotation starter, bet it just before the game.
5. In football the easiest rule is bet favorites as close to open as possible and on dogs as late as possible.
6. If you are betting a national TV game total, especially an MNF, think about the total. If it is a "low" total or "high" total, ie one of the highest or lowest on the board, assume the public will bet the high game over, low game under as the day goes on so adjust accordingly.
Just remember if you are prepared to bet overnights or as early in the week as possible for football you are ahead of 95% of the crowd so that alone is an edge.