The theory was that the threads in your press is too course, so when you tighten down the lock down ring, your sizing and seating dies will not be perpendicular to the shell holder.

So, gun writers have wrote articles telling folks they need to square them. They offer no data on runout before and after, but it gives them something to write about. You basically raise the ram until it touches the bottom of your sizer and lock down your ring. For seaters, you put in a shim to touch the bottom of your sizer and then lock down your seater. The shim has to be square. I got a machinist friend to make me a shim that is flat within .0005". Shell holders themselves probably don't meet .001" spec. So go figure.

I find it makes littlerly no difference whatsoever. But I still do it. One of those things that won't hurt anything.