schools on this topic. Like Dodgeman stated, I used to always pre twist my wires with a good lineman's tool and snip off the top part of the twist so that all the wire would be in the actual wire nut. I did this forever.
Then on a job one week I was working alonside an electrician and saw that he only aligned the two wires, stripped down about one quarter inch and then stuck both wires up into the nut and then twisted.
His comment when I professionally challenged him was that the newer wire nuts are threaded in the copper top end of the wire nut. When properly inserted the threading will create a much tighter bond and one that will not slip.
I have been doing this every since. So, I guess that both ways will definately work and the key is to get a tight bond.
He did another thing that I have always done regardless. He did wrap the nut in black tape and took the wraping below the actual nut to reduce stress on the wires if they were pulled accidently. He said it was an insurance procedure only.
Like so many other things in todays construction world, many ways to skin a cat.
But the ruler of course is the inspector. You will want to do what he/she is looking for.