Since you have only a limited number of normal heroes you want to make sure that you make the most of the ones you have. Not all your cities need governors and if you don’t have epic heroes it will be better to keep most of your heroes together so you can duel with them easier and treat them with a nice banquet. Having your governors at level 100 will give your cities the best production bonus. If you have Epics or heroes with skillbooks that boost food production, you want to make sure that they are in the important cities with your governors. They do not have to be governor for you to get the bonus but only the highest bonus from a hero applies in a city.
So which cities should have governors?
Your Main city obviously should always have a governor. If you have any County or State capitals they should also have a governor. Your capital outpost and any 15c should be good candidate because they will produce more food.
Other way to increase the output of your governor ?
Go in your civil and assign them to positions that increase their politics. Equip them with equipment that raise their level. Raise their level, add skillbook with the farm skill.
What equipment should your governor have ?
The basic minimum your governors should have is a Cloud helm and Welfare of the People equipped and are assigned positions that increase their politics. If you don’t have welfare of People in hand you can always use a Cloud enforced lance which usually increase politic too.
What skill and skill books make good governor ?
At level 50 some epic heroes get a farm skill that give up to 25% food output. Sun Quan, Liu Bei, Cao Cao and Zhou Yu all have a 25 % Farm bonus while Zhang Cheng (from the hermit house) have only a 9% bonus. You can give skill book that have the Farm skill, this will increase your output too. Note that the farm skill from the heroes level 50 bonus and the skillbook stack together but if 2 heroes have the heroes level 50 skill only the best bonus from both heroes will be taken, same thing go from skill book.