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The SAAMI spec for .308 Win case length is 1.995"-2.015", and the military spec is 2.000"-2.015". Anywhere in there is fine. The 2.005" trim-to length is just the mid-point between the ends of the SAAMI spec. You don't need to trim again until a resized case exceeds 2.015" unless you are crimping, in which instance uniform length is needed for uniform crimp pressure and so has you trimming every load cycle.
With neck sizing-only there is very little case growth, so, assuming you don't crimp, you may have a long wait before you have to do it again. Most case growth occurs during a full-length resizing cycle. (See exaggerated illustration below.)
