I wouldn't ride over rocks on road tyres. One, rocks are unstable and uneven so you could upset your steering and fall. Loose pebbles/sticks on bitumen are the same; I avoid them. Secondly, hitting a rock with road tyres can cause pinch flats. But if it's smooth enough to be comfortable as in tiva's example (so assuming it's not deep/loose gravel), I would ride rather than walk - and clean the drivetrain of grit afterwards.