The optimum fraction will be different with different subjects, and level of detail, and lighting, and I'm not sure what else. Sometimes I think that phase of the moon must factor in somehow.
That said, I expect that you'll pretty quickly figure out what works for you under various conditions, and how hard you're willing to work to get the optimum step size.
Adalbert's approach seems to be simple and effective for what he does: just use DOF/3. That would be a waste of resources for what I do, but on the other hand his results are gorgeous while mine are utilitarian. Horses for courses...
--Rik