I need to trim some oak limbs from trees around my roof.  One limb, about 3-4 inches in diameter, looks to have a sort of natural reflex deflex shape, about the right length. Is it practical to attempt to rough out a stave maintaining the general shape, or should I stick with a relatively straight piece and shape it while drying?

I'd say go for it.

If its clear and free of knots and branches it should make a nice r/d bow.

You can rough out a bow from the limbs but it will depend on whether the shape you want is on the compression or tension side. I prefer starting out with a relatively straight stave and form it to my liking if it is practical.