Of the maybe 10 ribs cooks I've done, I've had to use the rack twice. It could be my inexperience w/ the rack, but I prefer laying them down. Just seems more natural!
Also, it obviously easier to baste if you want.
The ribs usually shrink enough by the end to lay flat if I have the room.
hopelessly, helplessly, happily addicted (to a shipload of Webers)