An engineer once told me that the anhedral on the F-4 tail was to help with low-speed yaw stability. The tail of the F-4, like most airplanes, generates negative lift, so anhedral on the horizontal tail would actually work like dihedral for a wing (it's stabilizing). This seems to imply there is a tradeoff with yaw stability while inverted, which of course wouldn't be a problem for the F-4.

As for RC aerobatic airplanes, especially so-called 3D, I doubt it has any benefit. The anhedral adds effective side area to the tail, which is the same as adding more fin area. It does add it down low, however, which might help knife-edge tracking if it is done properly, but that's only theory, and I believe that was the thinking behind the Curare tail. I remember some discussion about it in the '70s (or was it the '80s?). It seemed to work, but then so do many other designs without anhedral. You would probably have to be an aerodynamicist or an avid experimenter to design it so there is any discernible benefit.

If you think it looks cool, a little bit probably will not hurt. Or if you think you need to redistribute the vertical stabilization area lower, it might help.

As for adding maximim side lift capbility, there would be no benefit at all, since adding tail area is stabilizing. Good knife-edge lift has a lot to do with the proper balance between area ahead of the CG and area behind the CG. All else equal, adding effective fin area, without changing anything else, will make it harder for the rudder to yaw the airplane and generate side-lift. Also, the length of each horizontal stabilizer/elevator would have to be multiplied by the secant of the anhedral angle to keep the same pitch effectiveness, which would of course increase weight slightly.