Try not to get too hung up on sharpest apertures and diffraction softening though. Yes, you can see a difference when pixel peeping, but after you stop peering close up at a small section of a 5 foot wide image, then apply normal PP and sharpen for output, then view the resulting image at the final size (whether a 10x8 or as a digital image that will fit on a monitor screen) you are unlikely to see any of that slight softening.
If you start picking f/8 all the time, just because it is your sharpest aperture, you will lose out on using one of the more creative controls in photography, your aperture. I don't worry about diffraction etc., I set the aperture which gives me the DOF I want for the shot, that is far more important to me. If I am shooting a portrait with that lens, I will be wide open for subject separation and better background blur. If a landscape, I will likely be well stopped down to get the whole scene in the DOF. I don't care that the whole scene may be fractionally softer, I would much rather that than have all the foreground detail lost completely in out of focus mush.
Sometimes, I just don't have the option of using a wider aperture. Shooting aircraft or motorsports I need slow shutter speeds for propellor blur or background blur. If I need to use 1/80th second, even at ISO 100 that comes out around f/16 on a sunny day and sometimes I have to stop down even more. Again, I won't sacrifice my artistic aim of motion blur for a more static looking image, taken with a faster shutter speed forced on me by using f/8.
It's good to know about sharpest aperture and diffraction, but in practice I feel you need to stop worrying about them and just use the aperture you need for the image you want to create.