I have looked at the instructions for wirenuts from: 3M, Buchanan, Gardner-Bender, Thomas & Betts, and Ideal. All of the instructions allow for pre-twisting or no pre-twisting, it appears to be the installer's choice.
The problem with pre-twisting is not the final twist of the conductors. There are two areas of concern, first damage to the conductors done by the jaws of the tool. Second, the loss of contact between some conductors and the spring of the wire nut (for example a #18 ballast lead may be hidden in the twist creases of (2) #12).
