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There are practical considerations at work too. If you dial up the gain, your exposure time will be reduced and you’ll end up with a larger number of output files. This increases the amount of time, and storage required, to process through the calibration/registration/integration workflow.
Ray’s posts on exposure times for CMOS cameras should be a must-read. The key take home message is to expose for the suggested value above the bias/dark frames. This will ensure there is enough signal in each frame, and allows you to methodically judge your exposure time at a given gain very easily.
In typical urban/suburban environments, the light pollution is by far the largest source of noise. Even at low gain, the read noise is not large, and neither is the thermal noise.
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