Lots of good advice here. I have had both ,sizing dies and seating dies that seem inherently straight and some crooked. Sometimes you can play with indexing the dies slightly and get some help, some guys like to use a rubber o-ring under the lock ring similar to Lee dies. (never worked too well for me.)
Sometimes I have had luck with rotating the bullet while seating, sometimes it hasn't helped at all.
The one thing that I do find to be important,is that you get the bullet initially set in the top of the case as straight as possible. Don't count on the seating die to straighten it out for you.
Also when you set up you seating die, start by running a piece of sized, trimmed piece of brass up into the seating die and adjust the die down until the crimping ring of the die just touches the brass, then back the die out about 1/4 turn.
If you have a loaded round you can run it up into the die and use it to help center the die in the threads of the press before you tighten down the lock ring.
If you go to www.accurateshooter.com and do a search for "runout" you will get a bunch of articles dealing with the subject.
