Sounds like bottleneck rifle cases. When you don't have a honed die, removing the standard expander that pulls up on the neck as you withdraw the case and instead expanding to the final neck I.D. with the downward insertion of a mandrel has less tendency to tip the neck off-axis. If you are going to go to the trouble of using a separate expander, I prefer to use a Lyman M die to get the little step that lets you start bullets into the seating die straight which further improves on neck runout.
As Bart B. said, a case length spread of 0.010" won't make any discernable difference. The only exception that comes to mind would be if you are crimping you might want more consistency.