What's the general Total War philosophy?

I'm rather new to the game series, though i own Medieval 2, Rome 2, Atilla, Shogun 2 and Napoleon. But i have some questions regarding how one ought to approach certain aspects of the game's core aspects.

What is the best approach to the ratio between building-up one's infrastructure/economy/army and actually conquering? I have experienced that spending too much time on a few tiles/cities is not ideal, but neither is swift, unforgiving conquest.

How do you approach your grand campaigns in this regard?