Personally, I never shoot a common zoom at anything higher than f/8 due to diffraction. You can test this up against a brick wall, with the camera on a tripod and set in manual mode, focus point should be single, dead center. Start at your most commonly used focal length, doesn't really matter but for me at that range, I'd be at 100mm. Turn off stabilization. Expose image properly by getting the exposure to zero at all f/stops, choose your metering mode but make sure they're the same for all f/stops. Start at f/5.6, then f/8. then f/11, then f/16, then f/22. Compare the 5 images at a 100% crop, look at center and corners. You will more than likely see that f/8 will be the most desirable image in terms of sharpness, micro contrast, corner sharpness, and vignetting or light fall off around corners. You could test a few different focal lengths and make a chart so you know what to expect. Doesn't hurt to compare one with stabilization turned on so you it's effect as as well. Typically the short end and the long end will show higher degrees of diffraction. I have done this on all of my lenses but I am kind of weird like that, I would have tics if I didn't know my hardware limitations and strengths.
Lastly, image sharpness is not the end of it all, but for the scenes you posted, probably a good idea to find the sweet spot. In terms of portraits, you should be sure the focus point is on the eye, and consider for depth of field. If you shoot a head shot at 85mm, you will want to isolate the subject the best you can, test for this with an object of some sort. Many people, including myself, can get hung up on image sharpness but I have learned it's only a small part of lens quality, and image rendering.