Depends who I am playing but generally speaking I keep one or two 'real' infantry units with one or two missile units in each city on a neutral/friendly border or in cities that are unusually problematic. I deliberately avoid using low level, non-missile troops for garrison duty if I can afford better. In case a quiet neighbor gets uppity or I need a decent army in a pinch it gives me better piece of mind knowing that I can call upon respectable infantry units to help put out the fire instead of a pack of weekend warriors with pointy sticks.