If you're using
betfair exclusively then I'm not sure but if you have a load of accounts then the earlier you look at the prices, the better. Then you can jump on the bookies that are going the longest about your selections before the market forces knock them back into line.
As I (and Crowie, and probably others) have said before though, there are almost always better odds on prem games elselwhere than at
betfair. The exception would be if both teams are 6/4 ish where the
betfair prices can be much higher.