Strategically, I divide my total available turrets by the number of planets in need of them, then just spread them evenly all over the place. When I need more, I unlock some. This way, I have a uniform defence at every planet, the strengths and weaknesses of which are readily apparent by observing how other planets with the same setup deal with various incursions.
Tactically, I keep everything closely together so as to conentrate firepower, sacrificing the local resource in exchange for not bothering with wormhole defence or incurring AIP by having to kill warp gates in order to close down wormholes.
So, say that I have five planets. Every system gets two hardened and two unhardened FFs, 20 turrets of every type including Basic mkIIs and mkIIIs, 24 sniper turrets, 10 lightning and flak turrets and a counter-sniper turret just in case.
As AIP increases, I unlock more turret types in response to the AIs ship selection. But this whole FF/Turret setup is really just meant to repel small-scale attacks and harmless waves; heavy assaults will usually be ground down by roaming patrols, Dyson Gatlings, Golem-Generated Zombies, Human Resistance Fighters, or hastily-produced militias. I don't really put the main fleet on defensive duty unless I actually lose a frontier world or two.