As a standard practice, I trim my cases every time to the 'trim to' length in the manuals even though they may not need it as they haven't reached maximum length. Most of the time this requires only a 'dressing up' of the case mouth. From the accuracy articles I have read, because the brass does not flow evenly, squaring up the case mouth is supposed to improve accuracy. (Lyman manual "Reloading for Accuracy")