The issue with soft edges is not one of film flatness, but of field curvature. And - if you know what you're doing to can exploit it to do "impossible" things with depth of field.
Here's a few examples of what's going on, using a different box camera (Kinoflex):
Saab by
Berang Berang, on Flickr
Compare the bottom left of the frame with the top right. See it? That's field curvature in action. The zone of focus moves closer to the camera the further it is from the center of the lens. So really close objects will be in focus if they're in the edges of the frame - meanwhile distant objects, which would be in focus if they were in the center of the frame, will be out of focus at the edge of the frame. Check out the car, and the trees around it - same distance.
And now for the impossible
DOF Madness by
Berang Berang, on Flickr
Notice anything weird about the depth of field in this image? Look at the plant in the right foreground. Now look at the ground behind it. Now look at the center of the frame.
This is what happens when the DOF is curved!
In fact the only thing that you can do with the film to counteract this, is curve the film to match. That's why cameras like the Ansco Pioneer have extremely curved film planes.
