The whole point of having a structural ridgeline is to set a consistent sag in your hammock fabric. You don't want it to stretch at all.
Nylon cord will stretch. The amount that it does is dependent on a lot of things, though. You'd be better off using some cording that doesn't stretch (dyneema, etc).
“I think that when the lies are all told and forgot the truth will be there yet. It dont move about from place to place and it dont change from time to time. You cant corrupt it any more than you can salt salt.” - Cormac McCarthy