You can stop whenever you like. It's always useful, if you engage in war, to have a few spare settlers marching a few tiles behind your front line.
You'll probably raze a few cities, especially if you're using airplanes and bomb them repeatedly, slaughtering the population, or if you're trying to hold citites with 12+ pop. which will surely rebel in a turn or two. Sometimes it's better to create anew. If you're being attacked by fast-moving/penalty ignoring troops -Ansars, Keshiks, Riders, etc.- you can just burn your cities and leave one or two of them to be defended, then when your slow, heavy Pikemen have decimated them you can rebuild quickly.