I usually do 1-2 in ancient and medieval depending on the importance of the city (more if they are border cities and under threat). When I get to industrial I ALWAYS pump every city up to 3, with bombard units (usually captured ones) in critical cities or cities under threat. I also always upgrade everything ASAP, even the much criticized pikemen->musketman upgrade. If I don't have enough cash I'll go to 0% science for a couple turns to get enough funds.