All of those are ways to do it, actually...and sometimes it depends on which joint you are working on. However, in general, when working on....say...a Head tube/Top Tube joint, I tack the joint in four places....12, 3, 6 & 9 O'clock and then braze the whole fillet, in alternating quadrants. So...I might braze from Noon to 3, and then 6 to 9, and then 3 to 6, and finally 9 to noon. Alternating helps equalize the stresses and etc. You can also "tin" the joint, where you tack it, and then quickly braze all of the way around with a very thin fillet and then go back and do the thicker fillet that you might want. When doing a fillet brazed frame, I "tin" the seat tube to the bottom bracket shell joint, which allows me to make sure that the seat tube is brazed to the BB shell all of the way around before installing the down tube. I then go back and do a full fillet on the exposed part of the seat tube, when I do the Down Tube/BB shell joint.