I learned in various classes that F8 is the best for landscape shots, in some cases a bit more. F18 isn't the sharpest point of any lense, it acutally gets blurrier from F14 (or something around that) upwards. There is a term for it called "Beugungsunschärfe", but I have no idea what that's called in enlish.
True lenses have their sweet spot (but it differs per lens as far as I know), but on my "limited camera" I can't find any difference between f4-f18 on a 100% crop. I did really notice problems with the Tamron 90mm, but that was only when I got it down to f32. And I think the word is diffraction.