The image above should be viewed at "Original." (note: it's about 600 kB. Sorry for the large size, but I don't want jpg compression to muck up the study.)

If you've been studying photography and optical performance for any period of time, likely you've read that lenses perform best at their medium apertures. Of course, there are a few premium lenses that look rather amazing "wide open," but for the vast bulk of the lenses we use, the middle numbers provide the best performance.
So here's the proof. The example above is with the respected Nikon 35 f2.0 lens (AF, but non-D), but you'll find similar performance with most lenses. Each image is a one-hundred-percent crop from the image's center, so we're looking at the sharpest area of the lens's field. Pardon the white balance--or lack thereof--but that's not the point of this Washington frenzy, anyway. All images received the same amount (+1) image sharpening in-camera (D70), with no post-processing. Camera was tripod-mounted with available light exposure.
The point of this is to show precisely what a difference a specific aperture, or aperture range, makes when shooting. Look at how much shockingly better f4 looks than f2. So, unless you want a dreamy softness (and yes, you actually might), you should do what's necessary to get your lens in its prime performance area. Maybe even set your ISO to 400 instead of 100 (or whatever) and accept some noise, knowing that you've maximized the optics. Hey, Neat Image works wonders!
Of course, there is no denying that even if an image is soft wide-open, if that wide aperture allows you to capture an image that you might miss otherwise, then your aperture selection was a good one.
Back in the film days, few amateurs ever did resolution tests; it was too expensive and laborious. Now, we have no excuse. I don't regard any of this as pixel-peeping. It's learning to maximize the performance curve to take best advantage of what you have. Now let's go shoot some pictures.
Now, what I said at the beginning, about this stuff being common knowledge, well, that's true. It's just that I always kind of thought it to be theoretical, the stuff of arcane bench tests, as meaningful as "line pairs."
And for you macro fans: I strongly suggest that you try a similar test on your 1:1 jewel to see if you really can get decent resolution at f22-32. I'm guessing you'll find that f13 to f16 blow f22 away, and you'll never stop fully down again once you see your results. Sure, maybe you'll give up a tad of DOF, but the area of sharp focus will be as sharp as it can be.
And for a lot of what we do, isn't that precisely the point?
What's the phrase? "F8 and be there." Ohhh, now I understand.

Comments welcome,

Eric