[wilsonnb]

The effect of string stretching is pretty easy to verify. After putting on a new set, tune them to pitch and the stretch each one side to side and away from the fretboard a few inches. They will all be at least a quarter step flat afterwards, usually more. Tune them to pitch again and restretch and they will be less flat.

I'm not sure stretching matters for the long term tuning stability of the strings but definitely has an effect on the immediate tuning stability. Players who don't bend much and fret/strum lightly might not need the added stability but I like it.

Also, I've only ever heard the old strings sound better thing about flatwound strings, particularly on bass guitars, in which case it's true as long as "good sound" to you means a serious lack of treble.