Lenses do not go abruptly from "best" to total disaster. I realize that a few photographers are going to be super-critical and consider the "best" aperture to be "the only one acceptable" ... but then ... why have an aperture selection at all.
I personally will not hesitate to use any aperture I want to.
The "best" aperture will depend on the lens design criteria. Enlarging lenses are typically "best" in the center of the aperture range ... but the change from there to the extremes is ... let's face it ... not great.
Camera lenses - *may* be best in the "center", but high-speed lenses *tend* to be better closer to the larger aperture openings - under the theory of "that's where they will be used most often".
Guys... I would suggest that more confidence be placed in the Lens Designers - most of those I've worked with were *very* knowledgable and bright. They understand "diffraction" and a whole lot of other things - very well.