This has both advantages and disadvantages. It's great to have them close for resource transport purposes between your cities. On the other hand, If you are in an alliance and are attacking enemies on a regular basis, and your not near any of your enemies, it's better to have them spread out so you can join in on the attacks. You don't want to be an hour away from an enemy when you go into attack mode. They will know you're coming and have the opportunity to reinforce, thus killing your troops upon arrival.