I am not new to the sport of reloading, but as far as turning necks, that I've never done. I'm wanting to build a 257 WSM which would require necking down 270WSM brass to .257. At which point I think I would have to turn the outside of the necks. Am I correct? What is the best way to do this? Thanks for the help

Hey Bearcat. I'm over in Port Land. I'll tell you the little I know about it.
Whether or not you turn your necks is entirely dependant on the reamer used, and how it compares to your brass. That's why it's smartest to have brass in-hand, necked down to size before finalizing reamer measurements.
At that point you can have the reamer made to mate with the sized brass or such that the neck must be thinner to fit in the chamber. Your choice. One is a bit easier, the other more accurate.
When you size up or down (especially down, compared to fireforming, blowing out, to size up) the neck wall is never perfectly concentric. The brass must go somewhere, but it doesn't always agree with itself on how to distribute a .015" thick .277" diameter circle - say - into a .256" circle.
So, by sizing, then turning, you can control the concentricity much more closely. That means consistency, which means repeatability, which means accuracy.
Also, there is the small and maybe unimportant factor of bullet pull. A bullet snugged into a neck with a .017" thick wall will have more pull, and slightly greater peak pressure, than one with a .014" neck wall, AEBE. This isn't good or bad, just different. Greater bullet pull can increase velocities with a given powder charge, and less can keep pressures down with a fuller case, I believe is how that works, but don't quote me on that. (Or any of this!!)
One thing to consider, don't plan on a very thin neck (e.g., .011") when you grind the reamer, and then if you do feel you want to try less pull, you can always turn down half a thou or so for experimentation.

NOW, that said, if your smith already has a reamer you want to use, the simple thing is just to ask if you'll need to turn necks. If so, buy a neck turner and learn how to use it.

Seniors, feel free to grade me, or add anything I missed.

If you want to be graded, you get an A from me! Only thing I would say is that in necking down as is going to happen here, the very slight difference from 270 to 257 will hardly be noticeable. I doubt you'll even be able to tell the difference unless you're using a tubing mike graded in .0000". Even one whole caliber, like from 308 to 338 only makes about .0006" difference in thickness.

Try it first before you even consider any solution to a problem that may not exist.