The sharpening workflow uses three sharpening passes, one of which, creative localized sharpening, is optional.
Optimization for source and content can generally be achieved in a single operation. I recommend carrying out this phase as soon as all major corrections for tone and color have been done. Large moves in tone or color can increase noise, and distort or even wipe out sharpening, so the image should be close to final tonality before the initial sharpening.
In particular, if you plan to use the midtone contrast boost I described in the previous chapter (see "Midtone Contrast" in Chapter 4, Sharpening Tools and Techniques), I strongly recommend you do so before applying sharpening, because midtone contrast has a major impact on perceived sharpness, and can also exaggerate noise.
Optional creative sharpening can be done at any time after the initial optimizations for source and content have been carried out, but should be done at the image's native resolution prior to any resampling for output.
Sharpening for output should be done after any resampling to final ouput resolution. I do output sharpening as the last step prior to the color space conversion to output space.
When I print directly from Photoshop, I never actually convert the image to output space, letting Photoshop carry out the conversion on the print stream instead. When I deliver CMYK files for press, I sharpen as the final step prior to CMYK conversion. If final output size isn't known, I skip output sharpening, because the prepress operators will almost certainly sharpen the image after resampling. The images still benefit from the previous optimizations even if the prepress sharpening isn't absolutely optimal.
However, sharpening can have an impact on tonality, so don't be afraid to make fine-tuning adjustments to tone and color after applying source, content, or creative sharpening. Images that are well sharpened for output more often than not look hideous on screen, and my advice is to simply ignore the screen appearance.