One exception would be knob and tube wiring. Again that in it'self is not bad, but can't install insulation over/around it. Again that is not universal and depends on the local codes, but it is common limitation. Also in some areas insurance companies won't insure homes with knob and tube wiring.
But the basic reason for total rewiring would be if you are having mutliple problems in the house with diming/flickering lights, none working receptacles/lights etc.
Also if when you are doing extensive remodeling and walls are being opened.
Otherwise, in most case work is limited to repairing specific circutis that have problems, running new circuits as needed, and upgrading service entrance and panel.
There's no reason to replace wiring just because its old. Other than an underrated ground wire, 1946 wiring works just fine. So long as its not giving you trouble, I would leave it alone. Just becasue its new doesnt make it work better. :)