The step inside the needle valve body is right next to the outlet holes. This really goofs up the metering as the flat passes the holes. I take a drill that is a tight fit when entering the body from the needle side and drill down the step about 1/16" deeper. this allows the fuel to find its way more equally to each of the outlet holes. This also means you will be screwing the needle in further engaging more threads.
That's great! I knew that the position of the flat made a huge difference in how it needled (and why 5 separate needles would all work differently, depending on the conditions), but never diagnosed why it was like that. The suggested modification certainly does seem to address it, too.
Aside from that effect, I think the "cylindrical needle with a flat" was generally superior to the "full taper" needle, because the cylindrical portion was a close fit on the ID of the seat. That supported the needle on both sides, threads and inlet side, just like the old "+" sign needle. The full taper solves the problem you are trying to solve, but leaves the needle cantilevered by the threads - which just makes them even more prone to getting pounded out by vibration, just like the McCoy. Since they went to the full-taper, they had to start making the threads longer and the entire assembly out of steel, rather than the 60's brass version.
I always used the "cylindrical needle with a flat" version, and then selected which particular needle to use from a generous supply of them that I bought (when they were 75 cents a piece...) and which one seemed to work for a particular fuel/air temperature/prop (depending on where the flat wound up when it was otherwise letting in about the right amount of fuel). The saving grace was that the entire adjustment range in the summer was about 3 clicks (one "right" setting, and one click either side), so a particular needle would be good for a wide range of conditions, unless you changed something.
Of course, it never occurred to me to switch to the old, small, OS spraybar, which is far superior, and in any case, they were very hard to come by in central Kentucky in the mid-late 70's.
Brett