You guys got my interest again.

Anhedral stabs go back to bottom mounted non-adjustable (incidence) one-piece wing designs and AMA style pattern. Those small fuselages had little side area to carry knife-edge and the faster the airplane the better. Everything is different in modern turnaround and nearly all designs have two piece centrally mounted wings, which I don't believe are as sensitive to dihedral or vertical placement.

I often thought anhedral stabs were incorporated to effectively reposition the stab (vertically) after the fiberglass fuselage mold was made. To fly right several other design criteria needed to be in line with the designer's setup and flight trim such as weight and CG location, not to mention straight building practices. One man's nice flying design may be another man's headache.

Roll couple is mostly a wing dihedral issue but the flying height of the stab relative to the wing noticeably influences pitch couple. Altering wing incidence, CG location and down thrust also contribute to pitch trim and coupling with rudder application. High yaw requirements of FAI offer additional design challenges I've not yet had to deal with but I expect the fuselage side profile also contributes greatly.

For what its worth
Jim