Unless you're a professional lens test-chart photographer (and then I pity you), don't worry about it too much. Even the term sweet spot could mean different things to different photographers. Manufacturers always have to balance between resolving power and gradation, as you can't optimize for both at the same time. High resolving power comes with low contrast and vice-versa (high contrast comes with low resolving power). Let the lens designers worry about it.
From a physics standpoint, nearly all modern lenses are diffraction-limited once you get beyond f/8. Unless you need the DOF there's no reason from a sharpness standpoint to stop down beyond that aperture. Likewise, there are very, very few lenses that are sharpest when used wide open. But so what? If shooting wide open allows you to overcome camera shake then I guarantee that the image will look sharper than if you had stopped down to where the lens supposedly performs best.
The only way to really answer your own question is to get the bench-test results (MTF and other charts) for a given lens and read it. After you've done that you should crumple it up, throw it in the trash, and go out and shoot. Working on your composition and reminding yourself to not stab the shutter release with your finger will be a much better use of your time than reading lab reports.
-Chip
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People who do stupid things with dangerous substances often die! -me
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