OK - let's get one thing clear. Sharpening does not actually sharpen an image! It just creates the illusion of sharpness by increasing edge contrast, which makes an image appear sharper.
In order to get best results, all digital images, whether from a digital camera or scanner, will require at least some sharpening before they go to print. This is a normal part of the process of reprographic printing.
The amount of sharpening required depends on several factors, such as image size, imagesetter resolution, etc. The preferred method is called Unsharp Mask, which sharpens only edges of objects in an image. Sharpening is best done only once. If sharpening is applied to an already sharpened image, unsightly artefacts can appear.
Despite the strong temptation to give your images that extra bit of "punch", it's best not to sharpen at all unless of course you are working under instructions from the repro bureau or printer, or are producing prints direct for a client. Leave it to the client/printer to decide how much sharpening is needed.