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This is correct, under a number of assumptions.
1. that you ambient temperature is stable. If temp goes up or down then you are stuck with only a short number of darks. Do not combine darks from different temperatures.
2. Dark frames contain noise. Dark Current is not noise. It is unwanted signal. It is not noise because it is repeatable, it is not random, therefore by definition it is not noise. But there is noise in a dark current. This is why you need multiple dark frames. The more dark frames you have to combine the greater the signal to noise ratio and the less effect the dark 'noise' will have when calibrating your lights.
3. In a light polluted situation as in a city dark current noise is basically swamped by unwanted light "noise" (again it really isn't noise because it's not random). The lower the signal (ie background of a dark sky) the more noise will predominate. In a light polluted environment the light noise predominates not the random elements of the dark current so you can get away with far fewer dark frames for combining.
Bury and Brunell have a great chapter on this topic in the AIP4WIN book which I think should be on every astrophotographers bookshelf.
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