I've done both though shoot the vast majority on the wing. The Feds are silent on this issue though some states have laws against the practice while others allow it. One needs to check the state regulations to determine if the practice is legal or not.
The most common times to shoot a bird "sitting" is geese in the fields and sculling. In the first it is often a single bird that snuck in at some point - usually when I'm taking a nap. These birds do not like to flush and are such easy targets when they do that I see no difference in shooting them then or when on the ground. Actually, there is a difference- it is easy to head shoot a goose on the ground but much harder as it tries to fly/run away.
When sculling, one is often chasing divers and they are often loath to flush from a piece of debris floating down on them. The birds will either swim in front of the boat or off to the side but often not fly. Sitting up and yelling, pounding the boat, or otherwise causing a scene is more likely to cause the birds to swim faster or dive rather than fly. Shoot one though and the others take to the sky.
