the efects on the thrust side of the tail rotor is more critical than on the "in bound side" as the air has yet to be accelerated or reacted on the blade surfaces.
Its just going with the flow......./''''''''

Acctualy the main work of a tail rotor is done in stationary flight...while, in speed flight, most rotor crafts ( especialy coaxial main rotor  kinds ) uses tail fins like airplanes.Also most tail rotors are propulsive ones ( the stream of air they accelerate has grater ratio of speed aft than before the rotor plane ) so they need "free" space ( no obstacles - like tail fins ) to accelerat the air.
sorry for english

What makes more of a difference than the side of the tail boom is the direction of rotation - top going aft appears to be more efficient in terms of producing thrust in all relative wind directions.
The other problem is blockage of the airflow either onto or out of the tail rotor.

Interestingly, a fin downstream of the tail rotor increases the TR efficiency with psuedo ground effect, to the extent that Ray Prouty mentions 94% power need vs. 100% without a fin.

Has anyone taken his class BTW?  I plan so go either in April or September.

Hans

A tail rotor fitted on the left on a helicopter with the main rotor turning anti-clockise will be a pusher one and  one fitted on the right hand side will be a puller tail rotor  

I would imagine that the best compromise is the fenestron system (i.e. rotor inside rear fin). This would allow an unblocked airflow, and allow less drag in foward flight. In this case direction will be less critical, but top rearward will interfere less with the main vortex I would imagine.

One of the most critical factors is tail rotor height wrt the main blade. Since the main blade has to overcome the side thrust of the tail rotor, any height difference will cause a left down roll torque (since tail rotor is lower). Watch a jet ranger next time it touches down. A virtual dollar says it's left skid first...

Mart

kis1 gave the answer I think. I haven't heard though of a puller tail rotor. so considering that all are thrusters, left or right depends on the rotation of the main if its clockwise or anticlockwise.