I have a few hundred brand new Nosler .260 brass here (the beauty of Nosler being semi local) to load up, it says ready to load but should I treat it as clean used brass and go through the whole process?
I would say SIZE. Even the best brass (Lapua) has some dinged up mouths. I've run brass right out of the box before, but to do it right, you definitely should size and deburr the flash holes.
I have run quite a bit of Nosler brass over several chamberings and found it good to go.
I would definitely not size it. Nosler's packaging is very good and keeps dings to a minimum.
If you have reservations about the necks being round push them over and expander ball but not up into the shoulder area of the die.