Don't really know, but here's a stab.
I remember someone telling me that the Bell 212 tail rotor went the other way to previous models (eg 205) because it was more effective aerodynamically to have the tail rotor blades 'chopping' up into the rotor wash rather than rotating back and away from it.
It seems to make sense that if the advancing blade was the one at the top of the disc, it would maybe get faster relative airflow in forward flight.
Could be just crap, but it sounded convincing to me at the time!
[This message has been edited by Arm out the window (edited 23 March 2001).]