In my experience it's almost impossible to survive solely on lvl 10s, because of the cost to lumber and iron. At one point I was hitting over 200 lvl 10s a day and selling 1B food in order to buy resources to replenish my losses, and then had to qeue 3-4 times at a total of 9-12 hours just to get back what I had killed off, it ended with me being in a situation where I was gradually killing off my army, just in an attemp to feed it.
It is my recommendation that the best way is to have 1 or 2 cities surrounded by lvl 10s and the rest of the cities surrounded by 5s. At least this is what has worked for me.