They do stretch a little over time. I have kite flying lines from Dyneema and Spectra, and they stretch over time and need to be equalized (move the knots so the lengths match).
But it's very little, I had about 2.5cm change in 25m over a few years. That's 1 part in 1000. This is from irregular kite flying, so not under (very) high tension or constant stress.
