As taking a dark-frame shot is simply a matter of putting the lens cap on and pressing the shutter without changing any settings, this is not a problem.
Only raw images for the same camera model can be used as dark-frames, preferably taken around the same time as the photo they are being subtracted from. A method to mitigate this effect is to subtract one (or more) shots taken in the same conditions, but with the lens cap on. In long exposure shots (more than 1 sec) the non-homogeneous thermal noise becomes evident, mainly due to unevenness of the sensor and surrounding electronics. It is not effective against high ISO noise because of its different random nature.
Dark-frame subtraction is a method of dealing with thermal, dark-current and fixed-pattern noise.
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