If you sharpen an image file too early in its life cycle, what you may find is that down the road, after you make additional edits, or once you resize the image file, that sharpening may not be sufficient or, in some cases, may actually be detrimental.
For instance, I recently had a client come back and request a larger version of a previously delivered image file for use in a large display. Rather than going back to the original RAW file and making all of my edits again, I made the mistake of trying to resize a TIFF file that had already been sharpened. When I enlarged that file, the sharpening was detrimental. It created jagged edges and artifacts. When I finally went back to the beginning and sized the file appropriately before sharpening, the image quality was excellent. And the sharpening was tailored to the file I delivered.
Even if you’re not going to enlarge an image file, sharpening too early is still problematic—even if it only means the sharpening is ineffective. The most common instance of this in my own workflow is when I create a web-resolution image file for a client. I sharpen the high-res final—which, I should point out, is always done by looking at the pixels at least at a 50% enlargement for an accurate look at the overall effect, plus a once-over at 100% or more to check for any oversharpened edges—based on its full-size print resolution, and then I frequently resize that image file to create a web-ready duplicate. With this smaller file, if I don’t re-sharpen once I’ve shrunk the image, it won’t look as sharp as I’d like. In practice, it means that if you take a shortcut when preparing smaller image files for the web by skipping the web-specific sharpening of that downsized file, your image files won’t look very sharp.
It’s important to understand, too, that you need to sharpen according to the type of finished output. Images destined for print will likely require more sharpening than images destined for viewing on screen. It’s also helpful to know that the more ink is likely to bleed (in newsprint, for instance, as opposed to a glossy inkjet print) the more sharpening will be required to maintain an appropriately sharp appearance.
Whatever technique you prefer to use to sharpen your image files—whether it’s a combination of clarity and sharpening in Lightroom or one of Photoshop’s powerful tools like Unsharp Mask and Smart Sharpen—the most important step you can take to sharpen correctly is to wait until your image is finished before applying sharpening that’s specific to its output.
For more information on sharpening techniques, take a look at this tip: http://www.dpmag.com/how-to/tip-of-the-week/all-about-image-sharpening-08-13-12.html