I have chambered through the headstock, in the steady, holding the barrel in a milling vice and turning the reamer (special circumstance), and have held the reamer various ways. Every method has benefits and suits certain circumstances. Every method can produced a perfect result providing the method is perfectly employed. For a brief period, I even messed with a reamer holder set up in the milling attachment so I could center it up perfectly co-axial with the headstock but soon decided life was too short.
When I started out, I threaded between centers with the faceplate and a lathe dog driving the barrel. Chambering was done with the barrel held in the chuck. When I realized that the tailstock was sitting low on the lathe I was using, due to wear of the ways near the head stock, I started chambering in the steady since this allowed the breech end of the barrel to sit at the same level as the tailstock. As I got more and more fussy, I realized that the only way to ensure that the center was truly concentric to the bore was to dial the barrel in and bore the center rather than just relying upon the piloted reamer. It then occurred to me, if I had the barrel all dialed in the four jaw chuck, I might as well do the threading there as well. Then, by making sure the tailstock was perfectly centered on the headstock, chambering might as well be done then as well and that, generally speaking, is what I do now.
Now, we all know, most barrels are not perfectly straight and, regardless of your preferred method, you have to make some compromises. I'm not going to get into the minor details here because I don't have the time (I have to go shovel some snow) but generally speaking, proper use of any method is going to result in a quality job.