The reasons to retouch in RGB for me are different.
1/ The RGB is a smaller file than CMYK if the size of the repro is the same. It is faster to work on a smaller file.
And reasons to retouch in CMYK are that color moves in curves are more subtle... you have to make a bigger move to achieve the same color difference as a move on an RGB file. Complex curve moves in CMYK are not so touchy to perform as they are in RGB. Editing the black channel can control deep shadow detail with more finesse than editing in RGB. RGB, CMYK, and LAB all have strengths and weaknesses for editing. The more fluent you become in editing in any of them the better your total range of skills will be.
Monitor display of CMYK is about as accurate as monitor display of any other type of output if the profiles are accurate. The display is never perfect but it can get very close. You treat the display a little like looking a Polaroids and using your experience with that medium to predict what final film will look like. Estimating CMYK edits using the info pallet and a good monitor is WAY easier than guessing the look of final film by checking a light meter and Polaroids. Photographers have performed the latter with great skill for years. It shouldn't be too hard to learn the former.
Bob Smith
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