Your statement implies that you can calibrate a compass successfully in an environment that has some local effects (ie, rebar) and still have acceptable mod values. And that you should calibrate pretty much at every flight.
I'm certainly not implying the latter, far from, I recommend recalibrating only if you have a good reason to.
As for the former, Im pretty certain you can do a 'successful' calibration that is location specific. Whether or not mod values will always tell you that, Im not sure. I wouldnt count on it.