I've never renamed units. Might be useful. I often don't rename cities, but there are times I do. In Road to War I put an A or T after the name to remember what unit the city produces - I dont' want to foul up my health with both buildings to make either type of units. For well-specialized cities I think putting some sort of shorthand note next to the name might be good. On an Earth map it's usually a good idea. I can't remember that Manchester is in south-eastern africa to pick up all the resources. I get the note that the enemy is near Manchester and then search the world to find the "darn" city. With 25 cities it gets tedious. I usually name the cities based on geographical location so I can find them easily.