As a new player, I understand that players can choose when to advance the Act Deck. In other words, once the requisite number of clues are collected, Investigators can decide to either immediately advance the Act Deck or--assuming Agenda Deck is not close to advancing--continue under current act in order to play additional cards, earn more resources, move to specific location, and so on. The choice to delay would be most applicable to the final act when Investigators need to be prepared for the big bad stuff about to come their way.
With that said, is there an optimal phase/time to advance the Act Deck? Maybe that's too general of a question and really more scenario specific, but I figured I'd at least put it out there.