Some benchrest shooters I know have a case mouth thickness of like 10Mil. It has alot to do with chambering and how tight your throat is. If you got a real tight throat, and you ammo is straight, and you have enough tension to hold the bullet thru chambering and recoil, then he's full of it. If your tension is so loose you can move your bullet with your hand or recoil changes your COL, you need more. Most of the time crimping can actually hurt your accuracy. With crimping you have a hard time getting consistent tension. I have had success crimping A-Frames for my 300RUM over non crimp. You got to try but most of the time its another step thats a waste of time. FL dies with 1st or 2nd fired brass has plenty of neck tension. You can get loose with collet dies. But this can be remedied with by polishing your collet down 1mil or so. Usually the lower tension the straighter you can load that bullet, I have found. 3rd or 4th fire, you can have some problems I believe. depends on the cartidge too.