Deciding on when to place a bet on a system match depends on the system. If the system doesn't have criteria that can change (betting percentages, wind speed, etc.), you can place your bets at any time without having to worry that the match will fall out of the system.
If the system does have variables, you may want to wait until shortly before game-time to ensure the pick still fits all the criteria.
You can see what type of filters each system has by clicking on the system and scrolling to the bottom of the page.