I usually have a worker-building era towards the end of the Ancient age. This basically involves one or two food-rich cities that can replace a unit of population exactly as fast as they can build 10 shields. The onset of a post-despotic government might be a good time for this, because irrigation allows more growth then.
However, most of the work in my empire is almost always done by foreign labor. What I love about foreign workers is that they don't count as your units so you don't need to pay for their maintenance. All of their work is a free lunch. Because of this, I very rarely raze captured cities. In a monarchy, democracy or republic, I immediately rush-build workers, wait a round, and rush-build more, until the town I took is size 1. Sure it costs some money, but because these workers count as foreigners, they'll serve you forever, and for nothing. If you raze a city, at most half of its people survive to become your workers. If you occupy and rush, you keep the city and put ALL of them to work (well, all but one).
While you do this, make sure you don't let the city grow, because if it does, it will add one of your own citizens, and that citizen will be the next worker produced. Then he'll ask for gold piece each turn for the next X-hundred turns. No, it's better to get two foreigners who are willing to do his work for free.
Workers are always useful, even after all your infrastructure is built up. Let them alternate between cleaning pollution and farming forests.