Some aircraft have built-up tail surfaces, which need tail bracing wire. The AMR 42% Super Decathlon shown in the photo, has a built up tail like the full-scale Super Decathlon, and tail bracing is needed! First, check with the instruction manual of your aircraft if tail bracing is needed. Otherwise, if you feel that there is a considerable amount of twist in your tail section, tail bracing can’t hurt!
Many airplanes need bracing for strength. For example, many biplanes need functional rigging to keep the wing panels from departing the fuselage during stressful maneuvers. Similarly, some tail surfaces are the same.
Sullivan Products has a Universal Flying Wire Kit, which contains cable that is rated in excess of 100 lbs tensile strength as well as a few different mounting methods. I prefer to have my cables adjustable from all points where they contact a given surface. Over time, if the cable flexes, or loosens up, one can make the needed adjustments so that the tail surfaces are always rigid.
To make the cable adjustable, you can pass the cable through a coupler and solder a small section of unthreaded coupler to the end and use a clevis to attach the cable to the mounting bracket, or, attach the cable to the bracket by using crimps (like you would set up a pull-pull rudder assembly).