All Furniture on the rug
If space allows, placing all furniture legs on a rug will ground the space and give the appearance of balance. In a larger space or open plan room a large rug can create a zone. A good tip is to determine the furniture placement prior to selecting what size the rug should be, laying out newspaper on the floor is a good way of sensing scale.
Front legs only
This approach really works when the space is relatively small and ensures the rug connects the furniture together but maintains a sense of proportion. It also works for dining rooms where the space is restricted.
Everything off the rug
Small spaces often need to be treated differently to ensure a sense of balance is maintained. This is also applicable if you would like to introduce a pop of pattern or a bold design.
As already mentioned, this can be subjective and is often open to interpretation. A corner or oversized sofa might take up much of the visual space in a room and as such, a combination of off and on would work better.