Your dealer provided half the story. Some arms disengage A/S until the stylus touches down, but on Rega's A/S is always engaged (at whatever level the slider's at). Since the arm is always being pulled outward it has a tendency to drift until touchdown. Some resist better than others depending on friction, as Jrtent noted.
Swampwalker and Elizabeth provided the rest of the story. Setting A/S much lower than "equal to VTF" will reduce cueing drift and often provides sufficient anti-bias to track real world records properly.
I strongly agree with Elizabeth's suggestion (start with the slider at zero). As Jrtent described, on many Regas even a zero setting still results in some A/S being applied. This may be all you need, but it's cartridge specific so user experimentation is required.
One clarification: don't listen just for channel balance; A/S doesn't always affect that. Listen also for R channel mistracking on difficult-to-track passages. If you hear that, nudge A/S teensily higher until it tracks both channels equally. That's all the A/S you need, using more often does more (sonic) harm than good. (Note: if BOTH channels mistrack you should increase VTF, not A/S.)
