One often sees that said, but CMYK was (and still is at times) colour separations for the use in non-POD, non-bubblejet/laser printers. POD machines do not use printing plates. Granted, for colour they do use CMYK (and black) cartridges, but in a single pass, not as individual plates in four or more printing processes.
What I use to create my images is simply set to 'millions of colours' and I work on and Save as (for my covers) jpgs. The art/photo prog I use is capable of setting an existing image to CMYK, and if I ever try that option it literally separates the colours (just those four, and black) in to individual images. An interesting effect, but unnecessary!