ALL lenses (that I've ever tried or heard of) are at their optimum design performance at around 2 to 3 stops above maximum aperture. So if you are looking for best sharpness, contrast, and minimal vignetting, you should set it to this optimal setting.
In whole stop intervals, here is the list of f/stops:
Therefore, an f/2.8 lens will perform best at between f/5.6 and f/8. A typical 75-300mm lens will have a maximum aperture of f/5.6 at 300mm, so the optimum f-stop will be around f/11 or so. But at 75mm, if the lens can open up to f/3.5, then the optimum will be around f/8 at that focal length.
This "gate" of 2 or 3 stops can be wider for high quality lenses, and will apply more firmly with cheaper lenses.
So ideally, you should be able to set the camera using the aperture-priority mode to the ideal f-stop, and let the speed be determined by the camera.
Of course, you will have to monitor the speed that results. If you have stopped down your lens to f/11, and try to take panning shots at a resulting speed of 1/60th, you will get blurry pictures. You then have several choices:
1) Restrict the use of that camera/film/lens combo to static shots.
2) Use more sensitive film to achieve more speed (this will have the worst impact on your pictures, unfortunately).
3) Open up the aperture a little to try to get a bit more speed, and see if the resulting loss of image quality is acceptable, or even noticeable (as I said, a well-made lens will have a wider "gate" for you to play in without penalty.)
4) Get a faster lens.
Interestingly, I've noticed that my camera setup (Canon EOS 1v and Canon 'L' lenses)is intelligent enough that the camera, if set on 'P', seems to detect the lenses capabilities, and always stays around 2-3 stops up from maximum (i.e., within the "gate"), and only goes outside this envelope if it is really bright or really dark. Smart camera. I haven't run the experiment with my older/cheaper camera bodies (Elan II and Rebel) to see if they do the same.
The only thing you should feel when shooting a terrorist: Recoil.