Tony, the way I see it is this..
The reason I'm having to alter the levels & curves of the images prior to combining them is really a side effect of CCDSoft's limitations. If CCDSoft could combine the images and save the resulting image as a 32 bit FITS file, there would be no point doing it before comining the colours.
However, the resulting image from the combine in CCDSoft is not a FITS file, it's a BMP/TIF which doesn't hold the same bit depth as the FITS files.
Because the resulting combined image is only 16 bit at the most, the ability to get the faint detail in that combined image is limited - it's been chopped off in the bit conversion from 32 to 16.
So to counter-act this, I need to adjust the levels and cruves such that the information I want is within the 16 bits that will be left. Then combine the images resulting in the 16 bit combined image, and not losing any of that faint detail that would have been chopped off.
Perhaps there are other applications (MaximDL? ImagePlus? AstroArt?) which can combine FITS files resulting in a 32bit file. I would expect in this case there to be no difference in doing the levels etc before or only after combining the LRGB.
Perhaps I am right (could be wrong), and perhaps that makes sense
Roger.