You really just want to make sure your water is all constantly in motion. As far as surface agitation, you may or may not need any additional movement at the top. What kind of tank do you have? Is it reef-ready with overflows? If so, how many times does the tank turn over per hour? Likely if your return pump is moving enough, you dont need the extra movement at the top of the tank. Surface agitation does two thing, it provides gas exchange and prevents a film from forming on the surface. If you have a sump and a skimmer, you are likely getting enough gas exchange.
Water movement in the tank. There are several ways to provide flow. You can use a bunch of smaller powerheads in all corners of the tank to make sure water is being moved. One other way is to use one or more large pumps which are capable of moving great a deal of water. For example a large closed loop that moves water across the back of the tank will cause a gyre, and affect the water in the front of the tank as well. How efficient it is depends upon the size of the tank and the capacity of the pump.
If your tank is small enough, a pair of large powerheads on each side of the tank could provide the bulk of the flow you need. With just supplemental powerheads added where water is not moving well.
A good way to see the flow in your tank is to use purple up, and watch how it moves. You can just pour some in front of a powerhead and watch how well it moves across and around the tank. You can also draw a bunch into a turkey baster and squirt it in the corners of the tank where dead spots usually form. If the white liquid doesn't disperse quickly, then you have a dead spot, and require supplementation.
Keep in mind that you and your system will be happiest with a wave making system that provides easy control over flow level and randomness.