When you view an image at less than 100 percent size, Aperture smooths it (a technique known as anti-aliasing) to get rid of artifacts that result from scaling down the photo. The relationship between anti-aliasing and sharpening is a trade-off; although anti-aliasing does not modify the actual image file, it does counter the effect of sharpening when viewing a scaled-down image.
Therefore, you should use the loupe or the zoom toggle (Z key) to evaluate your image for sharpness, and to check your work after using the sharpening tool.
Other applications, such as Adobe Photoshop, handle anti-aliasing differently, so the degree of apparent sharpness can vary from one application to the next when viewing an image at a scaled-down size. However, you should see consistency when comparing the image at 100 percent across other applications.
Note: Masters may appear sharper than versions at scaled-down sizes. Learn more here.