DM
Every brass make out there will tell you to size all new brass. I do not know of any ammo manufacturer that does not size all new brass.
Why size it? For one thing to make it all as consistant as possible, just because it comes from the same manufacturer does not mean it is all the same lot number, and even in the same lot number there will be some variance. It all depends on the guy running the machine, how much did he have to drink the night before? Is anyone at home sick? Is he late on the car payment? Did his boss piss him off?
The whole point to re-loading is to make more accurate ammo than you can buy (yeah some cost savings) so why would you not want all your brass the same?
You will NEVER get all the "new" brass the same because it came to you "undersized" in the first place... It has to fit every chamber that comes along even the ones that are normally tight, so untill it's "fireformed" it's undersized and set up for min. headspace chambers.
I mfg. ammo for many years and i can't tell you how many new brass i "didn't" resize, and in all of those years i never saw even one problem from skipping this step. If the case is visally "ok" and you run an expander ball through the neck you won't have a problem...
I also formed cases for wildcats and sold that brass too... No fl. sizeing was needed before useing them either...
If you want to take the extra step, go for it... That's all it is, an extra step.
DM