The impression I got from colleagues was that CMOS bias frames, even when taken at the same sensor temp and gain setting, were not as reliably repeatable as CCD. To me, that suggested that collecting darks immediately before or after your imagery might be a good idea. Collecting darks on another night – less so.
Afterall, its worth noting that professional astronomers have been slow to take up CMOS and I rather doubt thats because they are all stuck in their ways – as some have suggested in the past.
I must admit, as a CCDer, I have never used bias frames because I only use 3 or 4 exposure settings and just redo those every few weeks – just to be sure.
For CCDs a dark of the same exposure length as the lights works fine. Throw in a decent flat field image and a defect map and its as good as it gets.
