Placement depends on the coral being placed. Some corals prefer high currents some prefer almost no current and there's always the in between. Lighting will also play an important part in placing...depending on the lighting of your tank and the needs of the corals, you will have to place some high near the lights and others may need to be shaded.
Also, research how the coral you want defends itself or stakes out territory. Aggressive corals will have nematocysts that can really pack a sting and kill other corals if placed too close. Some LPS
corals can send out long sweeper tentacles that can damage and kill other corals.
If you don't already have it, pick up a copy of Eric Borneman's book "Aquarium Corals" and try to research the needs of every coral you want and then decide whether its right for your tank and where it will fit in best in your system. HTH