🚀 go-pugleaf

To what Lou writes, I would add that if you use CMYK inks for grayscale or duotone images, you will achieve more-stable results if you use a profile with a high GCR (Gray Component Replacement) in your conversion. A higher GCR means that, by reducing the amounts of chromatic inks (C, M, Y) and substituting them with black ink (K) in areas of the image that ought to look neutral on press (or that contain a neutral - 'gray' - component), it will become far less likely that any slight imbalance in the printing process would cause an unwanted color cast in the final printed results.

You can build higher-GCR custom versions of standard ICC CMYK profiles by means of a professional profiling application like XRite's ProfileMaker.

If you do not have the profiling application, or access to it, then use the standard profile, but remember to watch out for color casts in your grayscale output on press.

Whatever you do, use ICC profiles and the 'Convert to Profile' command in Photoshop, including an appropriate choice of rendering intent and black point compensation -- and stay away from Photoshop's Custom CMYK engine, which I do *not* endorse in any way, shape or form.