Always bow and stern. For one thing most manufacturers require them for their warranty to cover damage to your boar, rack, and car if the rack fails for any reason. Doesn’t happen often, but it does happen.
For another, if you use the car’s OEM roof components with your rack and ever watch crash tests you will often see the crossbars happily tumble off of the roof as the body flexes. In an accident the roof rack or other components will almost always fail before the straps holding the boat. Bow and stern lines can often reduce the risk of the boat becoming a lethal missile and injuring someone else or damaging another vehicle in addition to destroying your boat.
The following is an example of two errors. The owner was interrupted when strapping down his boat and failed to correctly fasten the front strap. Never let yourself be interrupted when strapping down a boat. In addition he did not us bow and stern tiedowns. The result was art highway speeds the front of the boat rose up and tore the entire rack off of the SUV. In the picture the boat and rack have been retrieved from the middle of the road and are now leaning against the side of his van. Note that the boat is still securely strapped to the rack at the stern. Fortunately no one was following close behind.