In my experience, prick-punching surfaces for brazing can be much more effective than tack-welding the tube/pipe in place.
Too often the tack-welds are large, and, if not done correctly, can oxidize critical surfaces inside the joint, and often cause some distortion of the joint components, resulting in less-than-ideal clearances at the entrance to the joint being brazed. The prick-punch method has no such problems. It is a simple, clean technique, for which clearances can be controlled quite nicely.
Another option, somewhat similar to the prick-punch method, is the use of knurling, as shown in Fig. 4. By knurling the OD surface of the tube that is pushed into the fitting, it can act in a similar way as
prick-punching, with each vertical knurl on the circumference acting to keep the component centered in the joint, while allowing liquid BFM to flow between them.
Knurling can be automated, if desired, and, as with prick-punching, has no negative effect inside the joint whatsoever.
Bear in mind, too, that it is not critical that the gap be absolutely uniform all around the joint. The critical things are cleanliness of the joint, the correct amount of filler metal, and the good evidence of a complete meniscus (fillet) on each end of the joint when the brazing is completed.