where you put it will mostly depend on your aquascape and what you are keeping. SPS love the high flow, other corals, not so much.
In a 5' tank I run 2 on opposite ends, one more front and high and the other lower and towards the back. I get a circular flow around the aquascape that i make random with the programming (tidal mode, ect). I have also seen people with longer tanks put two at the back, so they get a circular flow from back to front along the length of the tank.
In a cube I use one MP10 on the side (cant be in back on the tank), a bit up high as not to blast the corals.
In both I have a return that I point into the MP stream to help randomize the flow.