Tacking and/or starting the full brazing along the tube/frame centerlines is generally thought to reduce the heat distortions that can lead to a twisted frame. By applying the heat at the center line and then trapping the tube together with a tack or the beginning of a full brazing traps/keys the tubes with less side ways uneven heat expansion (distortion). Once the centerlines are linked then working down and away from the centerline, a bit on one side then the other side, is the attempt to reduce the distortions and trying to be consistent WRT the heat applied on the tube sides. Andy