EF-D Focusing Screen Question

When I purchased my 40D about 1-1/2 years ago, I bought and installed an EF-D focusing screen. I am an architect and for me, having the grid is really important to double-check that I'm getting my lines composed vertically and horizontally (at least when that is my intent!)

My question is, just today I found that the 40D has a custom menu setting to be used when you are using this focusing screen instead of the standard focusing screen. What compensation does the firmware perform based on the focusing screen? I have always been happy with my results from the camera, will I actually see any difference now that I've set the camera to acknowledge the focusing screen change?