Trimming isn't necessary each time you size unless you are up to or beyond the max case length. I usually trim every time though to the trim to length stated in the reloading manual. Its just something I do for consistancies sake and to ensure the case mouth is squared up. And I've also read in my Lyman's manual on reloading for accuracy that this is a step for increased consistancy and accuracy. But, like I said, it isn't neccessary every time.