I practice both at different times. What bothers me is other electricians who arrogantly believe that I am not performing a quality installation if I don't pre-twist my wires. (not saying anyone here, to this point is doing that). To me it is all about attention to proper detail and personal integrity. On my jobs, I don't allow guys to use a drill to install wiring devices. But then again, I will make exceptions when someone proves they care about the final product. Twisting wire nuts adds one more level Or more accurately removes one more chance of a good installation, but is not necessary for a person who takes care to install them properly.