Hi Tom,
Thanks for the feedback. You have a point. But I feel there's no hard and fast rule to say why you should or shouldn't preferably stay f/4 or smaller aperrtures. Sometimes how large you want to go with the aperture depends also on the composition, as you indirectly commented (body of dog...) On a similar note, I used to think wrongly that in macro shots, you must always use f/22 to get everything in focus cos you're already so close. True for some instances, but exceptions always exist in photography.
Personally, I've shot f/2 on a telephoto and I get solid bokeh and the animal virtually jumps out at the viewer bcos of the very selective focusing. I don't care if the lens didn't perform as well as it can at f/4 or even f/8 cos the relative sharpness in the subject is telling. It doesn't matter that not all of the animal is in focus.
Of cos, it depends on how you want to portray the animal or subject as I mentioned above in terms of composition and my previous posting. For eg, if you want to show to your friends or others how adorable your pet dog is, you probably want more DOF and more body, in which case you want to consider a smaller aperture -- precisely the point you're making.
Much said, I'm a bit puzzled about your blurred shots bcos of large apertures. From my experience, it is possible for slight camera shake to cause an apparent blame on the large aperture used and lens not performing well. In which case you will see none of the subject in focus, especially the region outside the field of sharpness, eg the background.
But assuming you are sure it's not a "shutter speed too low" problem, then it's even harder to understand why your shots came out blur. No matter how large the aperture you use, there must be one point that is sharpest, which is of cos the point you focus on.
A possible reason I can think of is that the animal might have moved a little (rem DOF is very limiited at wide open). Another is that you didn't focus at the exact point you want, especially if you use AF or your eyesight is not properly corrected. (No offence but I've read a few instances where pple claim their pictures become sharper after they change their glasses.) Even then, you should get at least a region of sharpness in your subject instead of blur.
