Not sure whether you're referring to transport racks or storage racks.
For transport, my saddles are set up at a little over 6 feet apart for both the Oasis and Adventure (both right side up). With that much spread, there is no need to tie down the bow and stern. Most roof racks require a much smaller spread, but there are generally no problems as long as the bow and stern are stabilized.
For storage, the slings (or bars) are also about 6 feet apart for both boats (hanging upside down), mostly for convenience. For relatively even weight distribution you would probably want to plan your supports one third to one quarter of the boat length in from the ends.