For the 40 SW and 45 ACP, which headspace on the case mouth, a consistent length within tolerances is a must. Some cases may never need trimming because length grows slowly but still should be verified before using a progressive loader for these.
The 38/357, 44Spcl/44Mag, and 45Colt, because they headspace on the rim, may never need to be trimmed. I do trim mine when they get too long just to keep the crimp in the same place for that particular load with the same bullet.
Ken