Quote Originally Posted by JBrunner View Post
I would qualify that as starting at where the lens is made to be sharpest.
That's O.K.
But only with the insertion of the words "to show". As in " starting to show at where the lens is made to be sharpest."

Diffraction always increases with stopping down. Whether you will notice any ill effects depends on what at the time is limiting performance. The effect of aberrations may be far worse than that of diffraction.
But diffractions starts to reduce the maximum possible resolution the moment you use anything but the largest aperture.