Back to my post, you don't need any backpressure, however, it is a natural result from a properly designed exhaust that will keep the velocity up, and prevent the valves from being thermal shocked, and since it is unavoidable, as long as you are maximizing the exhaust velocity at the RPM range that you want to optimize [usually peak HP], you will have an acceptable backpressure in the other ranges [ignoring other issues such as valve timing, reversion from poor header design, etc, etc...].