Just always treat every hoof as an individual. And maybe the easiest way to make sure the coffin bone is ground parallel is to trim to uniform sole thickness, then you know that the bones of the leg are properly aligned. To make sure 100% you would need radiographs.

The shape of your shoes should match the shape of the hoof around the coronary band (looking down from above) because that reflects the shape of the coffin bone. Here is more in depth research on that and mapping internal structures without radiographs: http://www.e-hoofcare.com...t-of-the-foot-flyer.html

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance