Turret placement is ever changing. Turret type on each platform will continually vary. The best way to understand how turrets work and what they work well against is to use a variety and try various combinations. Make adjustments as you watch replays of defenses on your base. You can unlock and upgrade multiple turrets on each platform. It is certainly worth investing metal in the different types of turrets available for your platforms so adjustments to your base can be made easily. Always with turret placement the idea is to use the turrets to protect your defensive buildings.
A quick word on turrets, power plants, transformers etc. Turrets (not the platforms, the turrents themselves) can be upgraded without taking an exp. hit. If you are trying to stay at a certain level you can upgrade the turrets themselves without gaining experience. This is a great way to build up your defense (defender capacity can also be upgraded for no exp hit). Transformer boosts are divided between the turrets they are boosting. A single turret boosted by a single transformer will have a much higher rate of fire than a turret shareing a transformer. Turrets like rail guns and howitzers benefit greatly from this type of boosting. Protecting your power plant is important too. Once the power plant is down your transformers go to 10% health. Consider these factors as well when setting your base up.
Finally for turrets a quick rundown on each type.
Machine gun, best used for additional anti air as well as strong against light vehicles.
Howitzers, slow rate of fire unless boosted, strong against light and heavy vehicles. Good against infantry. Venerable to air.
Rail guns, slow rate of fire unless boosted, very strong vs heavy vehicles venerable to air attacks.
Flak guns, very strong vs any air units. Venerable to ground assault.
Salvo, expensive, flexible hits both air and ground targets. Good vs infantry and helicopters.