I'll take stab......I have the welders put in this optional notch. The purpose of the nick break for welder qualification is to examine the weld metal soundness. So I like this break to start in the middle of the face and run as true through the center to the root as possible, unless it is to follow a discontinuity. The Branch/Butt "Multiple" qualification test is usually done on .250" wt. More times than not, if this notch isn't present, the weld will tare off to one direction with a large portion of the pipe wall present in the specimen. We have some continuity tests performed on .154" wt pipe. Without this notch it never breaks through the weld. Also, ER70 are the minimums for electrode strength, pipe wall strengths can be much less.