Jim,
as George says, the smaller the aperture the greater the impact of diffraction on muddying up the images.

Most lenses have a sweet spot in terms of optimum sharpness and it's *usually* 2-3 stops down from maximum. Personally, I like to shoot at F8 or F11 for scenics because it gives me good depth-of-field and good sharpness without any diffraction effects.

There are reasons for closing a lens down -- usually when shooting macro or extreme wide-angle -- but for most lenses, try to shoot at the sweet spot.

Jim