Let's say you have exactly two Actions remaining and two Action cards in your hand. One is Library and the other is a terminal Action that doesn't draw. It used to be that I would always play Library first, hoping to keep my "engine" going, such as it was. Lately I've been second guessing myself on that. I've found that, assuming I have some decent amount of Treasure in my deck, I sometimes get better results by playing my other Action card first and then taking advantage of Library's ability to draw past Actions to my Silvers and Golds. Not surprisingly, this works best when the other Action card decreases my handsize by more than one, a la Remodel or Horse Traders.
Obviously this is highly board-dependent, etc., etc., but does anybody else do this? You know you could potentially draw more Actions, but go for the big Treasure payoff instead? Do you have a rigid Library policy, or is it more flexible?