So, I'm watching Rod Goodman's video on B&H about headshots.
He said the reason he shoots at 5.6 is that a lens is sharpest two stops from wide open, he shoots with the 70-200 2.8
Rod Goodman, while simplifying his case, is pretty much right. While QC is usually pretty good, being mass produced items, YMMV with any lens. My current 70-200 f/2.8isII looks sharpest at f/4.5-f/5. Others I've had may have a sweet spot at f/5.6. As a general rule of thumb, it's the middle apertures in the f/5.6-8 range that most photographers will report as being a lenses sweet-spot for maximum edge to edge sharpness.
I know it sounds crazy, but my new 24-70 f/2.8II is so good I can barely distinguish between infinity focused shots at f/2.8 or f/5.6. All pin sharp edge to edge. Extraordinary. Let's hope this is the new standard. It's definitely not the norm.
For the most part, few lenses are going to deliver maximum sharpness wide open or shut right down to f/16 or F/22. Some macro lenses may be optimized for good sharpness at F/16.
Prove it for yourself and run controlled tests, tripod mounted, with your different lenses. Lock onto the same focus point and run through the apertures in one-stop increments. Be sure you're shooting fast enough so there is no camera shake. Don't use AI focus for this test. Be very careful and sure with your focus lock. You'll learn a lot about lenses in general.