Interesting you ask that just now. I have been working on a TD-160, and the headshell was always drifting towards the edge of the record as the needle dropped. During the process of rewiring the tonearm, I had to loosen the right horizontal bearing screw in order to remove the tonearm. I thought to myself as I loosened it that it was on there awfully tight.
Anyway, when I reinstalled the tonearm, I tightened that bearing screw to (what I think) is a reasonable tightness. Now the headshell drops straight and doesn't drift. I think the proper adjustment is to tighten the screw all the way, then back-off a quarter turn. I went ahead and adjusted both sides/screws the same way.
The tightness of the screws may be totally unrelated to that headshell drift, but just thought I would share!