Well, actually I'm speaking from the perspective of the PoW, PoP, and custom scenarios where you often have the resources to get a crime system running early on. But no matter what the map type, it's best to get crime under control as soon as you can, and a cheap and slow punishment is fine to start. Plus if you choose one of the slower punishments, you won't need a torturer's guild. I would upgrade as soon as you can, though, so you can keep your production up.
Guard post placement is not that complicated when you understand the dynamics involved. The crime we're speaking of is theft from the granary. As far as I know that's always the case. A peasant that turns to crime first goes to a hovel and straps a small "booty bag" onto his belt and begins a "sneaky walk" to the granary. As soon as he does that the guards will recognize him as a criminal and will give chase. When they catch him they escort him to the dungeon for incarceration and further processing.
Many players like to put a guard post near the hovels. This puts a guard very close to where a peasant can first be spotted as a criminal. This can work well if all your hovels are in one area, but if they are scattered around, you would need extra guard posts. My preference is to put one at the granary, since no matter where a peasant is when he turns to crime, he will start heading to the granary--and the guard. This isn't quite as fast as having a guard right at the hovel, but isn't far off since the criminal and guard will be heading directly toward one another, and is a very safe solution.
So you can do quite well with one guard post if you wish, although very high crime rates or large distances may require more to keep up. If you choose to place one near the hovels, it's still a good idea to place one near the granary as back up. I don't believe there is any distance restriction on guards as there is for gong farmers and falconers.
Up to version 1.2 if you had a working inn, peasants would go there to start their crime, rather than a hovel, making a guard at the granary even more important. A test I did on 1.3.1 just recently seemed to indicate this may no longer be the case, but I need to do further testing to confirm this.