It always depends on what type of picture you are processing, and what you want to achieve in that picture.

• Are you processing a macro photo?
• A landscape?
• A portrait?
• How is the depth of field in the picture?
• Where does the attention of the viewer needs to be attracted?

All these question will influence the way you process your image, respectively the area and the amount of sharpening you are applying to your image.

It will also help to understand what sharpening actually is and how exactly is that achieved. Cambridge In Colour has a small and useful introduction to this theme.

Excerpts from the article:

Image sharpening is a powerful tool for emphasizing texture and drawing viewer focus. It's also required of any digital photo at some point — whether you're aware it's been applied or not. Digital camera sensors and lenses always blur an image to some degree, for example, and this requires correction. However, not all sharpening techniques are created equal. When performed too aggressively, unsightly sharpening artifacts may appear. On the other hand, when done correctly, sharpening can often improve apparent image quality even more so than upgrading to a high-end camera lens.

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In some cases, you want to use sharpening in order to give the feeling of a better focus in the image and thus also draw the attention of the viewer to the focused area. In this case, you would sharpen only the "in focus" area, like in this image, where the attention is supposed to be on the eye of the subject

In some other cases, you want to create the impression of a wider focus area and try to sharpen also the surrounding area. Take a look at the next 2 pictures and notice how the sharpening in the second picture is popping up also the basket, but leaves the area outside the basket still out of the "interest area"

vs.

On the other hand, if your image is a landscape, and the whole image is supposed to be in focus, then you would most likely want to sharpen the whole image.