I think it's a matter of taste. In terms of raw power, rushing the AI probably is best. On early, you can often get good quality troops while it is still fielding peasants. But that's a little lame. With some factions (Almo, Biz) have unique unit advantages which decline over time that reinforce that. Poland and maybe other small factions also needs to get going quickly or it will stall.

For normal factions, I tend to wait for a nice European sea trade network before I make big moves, but that is also rather exploiting a weakness of the game.

For a fun role-playing game, I take it slow - only fighting those who attack me or hinder my GAs.