I think that F-2 works greats for portraits in general...2-3 seems to be a good number when i m checking out the "family" portraits that i have taken, just give the whole image a very nice bokeh...
It's a personal preference as well as working with your surroundings. Quite often I stop my lenses to 5.6 for a compromise between optical quality and getting the right amount of DoF. Below is a random snapshot (it's not really a portrait, nor was it meant to be), but something recent that demonstrates that you don't need a lens to be fully blown to get that bokeh.
150mm @ f/5.6, 1/250, ISO 2200.