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.

In general, we try to advance the act with as many actions left this round as possible. That way if it causes something horrible, we can deal with it, maybe. If we're four handed and the last person to go gets the last clue, we'll try to wait until next round to advance.

There's exceptions to that of course, especially if we know what advancing the act will cause. Another case where we'd wait is if we thought we were ahead and wanted to get more VP.

I think generally speaking you don't want to advance the act deck if it's the end of the intestigators' turns. It is useful to have some actions left to respond to changes in the game introduced by the next act. That said, and as you suggest, it tends to be difficult to apply a rule. If it is the final act card, you may want to progress as soon as you can, even if that is the end of turn...particularly if you know waiting on another mythos phase to complete could be challenging. If it is still early, and you have the opportunity to hoover up some victory points, you might want to delay. So, no general rule per se, but having at least one investigator with 2 to 3 actions left in the turn, is generally better more times than not, than progressing with no actions available. Progressing with all investigators ready for action, is even better if you can make that work.

This helps a lot. Thanks guys!

I concur. There are also times when you have no choice. The Act deck advances automatically when you have finished whatever is required. Keeping that in mind, we have been known to put off doing that last thing to finish off an enemy with XP value, or to clear a location for XP, or even to have an investigator on his/her last legs, go to a location to be healed. As long as the trade off with the Agenda deck is reasonable.