Some people, in an effort to have the lightest possible car, make the rear axle as narrow as possible. This requires a minimal wheel backspace and results in a lot of bending stress on the axles. It would be possible to have near zero bending stress in the axle by having the axle bearing centered in the tire tread. I usually just put the axle flange near the center of the tread and run a centered wheel. This results in a bending moment of about 2.5", which isn't much. If you're running a wide tire and small backspace wheel you'd better keep an eye on it.
As far as the housing goes, the same physics apply but the distance from the suspension mounts to the axle bearing is what counts.