Why should the ball be centered during steep turns? I was taught to make steep turns with the ball centered, but this seems wrong to me. Shouldn’t the ball actually be held slightly to the inside of the turn?

My thought is that by using some cross-controlled rudder, the elevator pressure can be reduced, reducing the angle of attack and making the turn safer. 

Essentially, in a 45 degree turn for example, a standard tail aircraft is turned into a V-tail. Pulling on the elevator is only half effective in keeping the nose up, the other half of the force goes to turning faster. 

A knife edge is the extreme form of this, where the rudder becomes the elevator completely and the fuselage the lifting surface. No elevator is used (to keep the nose up. It’s actually used as a rudder instead).

So, if keeping the ball to the inside reduces G-force and angle of attack, keeping the plane further from the stall, why isn’t it commonly taught?
Coordinated flight, even in turns, will minimize drag.

Therefore, there is no efficiency gain by uncoordinated flight, in turns or not.

Therefore, rudder input should be as required to center the ball.