Rear derailleur problem....

[Ajax Bay]

braided cables

Not braided. Multi-strand long helix.

 

[raleighnut]

Don't braided cables undergo constructional stretch? Must Google

With any cable/wire there will be initial settling down often leading to more tension being needed but once that has occurred they should be fine.

 

[Ajax Bay]

initial settling down

Any initial settling down is down to cable housing ferrules/stops and the like and not to the inner cable 'stretching'.