I find it best not to build farms, at least not till end game.
I will have to respectfully disagree with that comment. If you play one of the economy-poor Eastern factions (Parthia, Armenia, Pontus), or the likewise poor barbarian factions (Germania, Scythia, Dacia, Spain, Britannia) it is absolutely essential to get your cities to grow to the point where you can begin to recruit your top tier troops. Otherwise, you will not be able to compete with the overwhelming power of the Roman factions.

For these factions (and you can throw Numidia into that mix), you need to build as many farms as you can afford as early as possible, and in some cases, build fertility temples to even further accelerate the growth process. You can slow the process down at a later date (by tearing down fertility temples, depleting the population with troop training, etc) and eventually reach ZPG (Zero Population Growth).

Just my 2denarii...............