I have to agree with Ralph that people tend to worry much more about diffraction than they need to. Getting the right exposure and the appropriate amount of depth of field for the image you want to make are much more important than avoiding diffraction in most cases.

One situation where diffraction can become a serious issue is macro photography at high magnification, when the effective f:stop is considerably smaller than the setting marked on the lens, and the gain in depth of field from stopping down the aperture is likely to be negated by the increase in diffraction and potentially other factors like camera/subject vibration with long or multiple-strobe exposures.