I agree completely with zoodiver on this, he has raised some very good points based on hands on experience.....no substitute for that!
I have been trimming my pups for a very longtime now. I usually do this when ever possible a week or two before shipping them out, during this time the protective membrane grows again surrounding the blunt stump that's left. All that is needed is a pair of snipe nose pliers (long handle ones)to hold the barb firm and clear of the body, a pair of electrical wire cutters is then used to trim the barb...oh yes and the confidence to do it! I catch the ray and place it in a shallow bowl to limit its movement, it usually takes about 30 seconds all in all. I too have noticed shippers in South America are doing this more now, its less stress for the ray because there is no need to remove it from the water. Many of the larger rays 30cm disc plus are man handled in rather a rough way by grabbing the stock (butt) of the tail with leather welding type gloves, this stops the ray from thrashing its tail whilst a sheath it applied, seen the rays going crazy whilst this practice is carried out.
So all in all I like the trimming method, reduces the chance of infection, I have seen a few problems with the sheath method, removed one off a new import and the smell from the dead proteins within the venom was over powering.
Other advantages is the newbie ray keeper has a few months to get used to his or her ray without worrying about being tagged, but most of all I do it because it prevents the bags being punctured whilst in transit.