I also must comment that I’ve tested turning the case after seating the bullet halfway many times, using conventional seating dies in many cartridges, and have yet to find it makes any significant difference in consistently seating bullets straighter. Evidently it makes a difference with some dies, in some instances, but I have never seen it.
I agree, I've never seen it make a difference either.
One thing I have noticed is unsquare case mouths are disaster. Often putting fireformed cases, and cases that are full length resized between each firing back in the case trimmer and squaring them up it is plain to see some case necks will stretch on one side more than the other, usually ones that have large neck thickness variations, this is evident in that it takes a turn or three until a few or often several thou has to been removed until finally cutting around the whole circumference of the mouth making it square. When seating a bullet on an unsquare case mouth the base of the bullet hits the tallest point of the mouth pushing it off center and tips it crooked, doesn't stop tipping it hits the low side of the mouth. This starts the bullet down the neck off center and tipped and stays crooked until it's seated all the way causing runout. Turning it in the shellholder as you seat it doesn't fix this. Add to that uneven neck thickness and it's pretty hard to seat a bullet straight in that scenario.
Personally when I want to make straight ammo I don't fiddle with turning cases in shell holders, and trying to straighten them after seating, I think that just screws up neck tension. To make straight ammo I turn necks and make sure the case necks are 100% square. When this is done no matter what seating die I use I get pretty concentric ammo, it would be hard not to.
