Just test it yourself. I answered this question for my lenses by taping a dollar bill to the wall, levelling the camera on a tripod, and shooting at apertures from wide open to about f16 over a range of focal lengths and at several different focus distances, then looking at the results on screen. Pretty much answers the question, at least for closer focus distances, and it's easy enough to test the same way in the field for longer distances.
Yes, of course. Personally I am a photo-hobby/enthusiast not a professional looking to create for-publication prints or wall hangings for clients.
Probably testing is the best solution. I asked here because in my photographic journey I am just learning about diffraction and how this makes lenses weaker at the smaller aperatures. So before I launch myself in to a 3 hour experiment I thought a few minutes on the ol'internet might garner time saving answers.
maybe a few examples will explain why i am asking.
For example the other day I saw an interesting pattern in my citiy's office towers. When I took this snap I choose the shutter speed of 1/320 (using the reciprocity rule) because I calculated it was the minimum shutter speed I could work with shooting at 200 mm on a crop sensor 30D.
This resulted in the largest DOF (my intention) but may have resulted in a f-stop where defraction caused a loss in sharpness. Now if I had known what the sweet spot was for the lens I might still have had sufficient DOF and had a sharper photo.
Likewise, when I encountered this shot I realised DOF wouldn't be an issue even if shot at 200 mm with the 70-200
or this shot taken at 70mm with the 24-70
So my think at that time was to shoot wide open at the max f 4 and f2.8 respectively so as to minimize possible camera shake. I didn't know that lenses were not as sharp wide open. If I'd known the lens' sweet spot then i could have selected that aperture (assuming it didn't invite problems due to camera shake).
I am just exploring ideas that, though new to me, have probably been part of the pros' mental calulations. If I can avoid a controlled testing of my lenses, then all the better.