I specialize early in the game, but as time progresses I don't worry much about it. How & where I do it depends on the faction. I ALWAYS have one building strat units; bishops first (for scouting), then ~6 assassins, then pump bishops while building to spies (then I put one spy in ea. of my provinces).

I also try to get 3 provinces building ships, though only onw is high priority early in the game.

BTW, I play mostly GA.