I use nylon coated steel fishing line for the cables and have never had a problem with the lines stretching over time. I've also use the nylon fishing line (no steel innards) on the smaller models (little electric jobs) with no problem with stretching. I suggest you use just enough tension so there is no sag at neutral, to tight is very hard on the servos and causes excessive servo gear/bearing wear and higher current drain. This has proven to work for me for tha past 20 years with no problems.