Single stranded wires were indeed a bad idea. No doubt they will be phased out eventually.
I have done current load testing on electrical accessories like switches and sockets where I ( electrician ) twisted the wires and my colleagues ( graduate engineers ) simply stripped the wire and shoved it in the terminal and tightened the screw until their screwdriver snapped. The temperature rise on the terminals with my twisted wires was always lower.
As for the mechanical strength of the twisted wires, if you don't nick or mash the copper with your pliers its all good.
I don't think it makes any difference with wire nuts because the wire nut twists the conductors as you tighten it.
Oh yeah PaulUK and Gloria, if you have seen a Conductor do the Watusi then he would certainly be twisted.
