The ideal setup will have the servo arm and control horn the same length, have the attachment point on the control horn directly on the hinge line and cables not crossed. If the control horn attachment point is behind the hinge line as is common then crossing the cables can help however the best fix is to attach the cables to the servo arm with the same amount of offset. The same applies if the servo arm needs to be shorter then the control horn. A tiller arm is a good idea and on larger models I would say required but in airplanes 100cc and larger. If correctly set up you really do not need a cable tension to the point of putting additional stress on the servo. In most cases it takes a little experimentation to get your geometry right but well worth the effort. Not saying this is the only way but simply what has worked well for me.