I've heard to never leave a knot in a climbing rope for an extended period of time as it can deform and weaken the core of the rope. I'm unclear if the same is true for static cord and rope as well.
I have two prusik loops which have been tied continuously for years and used as auto blocks. One of them is also part of my rescue kit, which means that on occasion it's used to anchor the rope and escape the belay. In this setup it's holding 100% of the climber's weight, rather than just engaging my belay device as it does when it's used as an auto block.
So I'm curious, should I be untying the fisherman's knot between uses?