1) You have to have the bullet straight. The seating plug is not going to straighten the bullet.
2) If you use a LOT of case mouth flare, it is LOT easier to set the bullet straight on the case and have it stay straight.
3) You can get a Hornady New Dimension seating die that has a sleeve that keeps the bullet aligned; however, you STILL have to have the bullet straight enough to go in the sleeve.
4) I use #2. I have never lost a case yet from over-working due to excess flare, and I would rather lose a case than make ammunition that is not as accurate as possible.
5) Get a Redding Profile Crimp Die for the roll crimp. Does a better job that any other roll crimp die I have tried.