Even if you have a good trimmer, it is challenging to keep the cartridge lengths to within .001. The purpose of the neck is to hold the bullet, and from a safety stand point it doesn't make much difference how long the neck is, although the more consistent the length is from round to round the better the accuracy you'll likely observe. Consider the .300 Savage and the .300 H&H. Both cartridges hold .30 caliber bullets, but the neck of the Savage is very short and the neck of the H&H is very long, yet both are successful cartridge designs. Occasionally, I trim '06 cases very short if the neck has been damaged, although I seldom use these cases for full powdered loads after that, but it is a uniformity issue rather than a safety issue.
If you are crimping with an RCBS or Redding seating die, consider crimping as a separate operation. If you simply adjust the die for each individual cartridge your crimps will be more uniform.
When I crimp I don't bother with the lock ring. Raise the seating stem to the top of it's travel, thread the die in until it makes contact with the case mouth when the ram is at the top of it's travel, then turn the die in another 1/8th of a turn and crimp. Because I don't use the lock ring I turn the case a half turn and run it up a second time to ensure the crimp is even on all sides. If your bullet has a deep crimping groove, you can actually crimp by turning the die in by hand until you feel it stop. I've found this technique works well with cast bullets.
