it is up to you. fish and corals enjoy current. remember, the goal is to mimic the ocean. You don't want to run the risk of blowing away your corals, but also don't want dead spots where cyanobacteria can grow. I have found that creating a vortex in the tank with opposing current makes for a nice, even flow thru your tank.
it also depends on your fish. My powder blue thrives on current. That is one of the reasons many don't survive in captivity. They are usually found in the wave breaks, with tons of current. In tanks, they tend to nervously pace back and forth and eventually waste away. I have two powerful powerheads on either end of my tank. plus the return pump. I find my powder blue floating in front of the powerhead at night, just surfing the powerful current. It is pretty cool to watch.
-Dr Marco :sfish: