I'm told that on most higher end acoustics some bowing of the top behind the bridge is normal and in fact, is likely to be built into the design of the guitar; making a flat top actually flat on the bottom but with a slight convex curve in the bridge area. Is this so; that a good acoustic's top should have a slight hump behind the bridge? If so, what's considered normal? Assuming there isn't a playability issue, do acoustic experts recommend doing something about it only when it exceeds a specific amount? What would that amount be?