I also have another 'take' on the twisting thing, speaking as an engineer, but have no scientific proof. I'd be very interested to know if others see it this way...
If you twist the wires you will have less overall contact area with the connection than if you just group them together in a straight line all in parallel?
And, as we all know, less contact area means more contact resistance = heat. (Which is bad).
Tony
I guess you're right. I think twisting prevents screws working loose from when the copper expands and contracts from heating/cooling down though (I wonder how many blackened cooker/immersion heater switches in the world have suffered from this)