The direction of spin is a contributor to the issue of tail rotor thrust, but it becomes important only when a limited marginal design is fielded. The power capability of the tail gearboxes, the pitch range of the tail rotor, its rpm and blade chord all conspire to make the maximum thrust what it is. When the tail rotor proves to be marginal, the easy fix is to flip it over and add that few percent of improved thrust back, since no components are changed (as long as the gearbox maintains adequate lube when upside down.) Any other substantive change to make more thrust usually means the entire tail rotor has to be changed, an expensive proposition.
In other words, flipping the tail rotor makes an unacceptable tail rotor now marginally better. Actually fixing it by creating more thrust would be better yet, but much more expensive.