How important is it to sort Lake City brass by year?
Depends on how good of a shot you are. If you can see a difference on the target between year groups, then it is important. If you can't, it isn't.
I did an experiment with mixed years verses all 07 during load workup, and I could not see a serious difference in group size or point of impact at 25 or 100 yards. With iron sights on a service rifle the groups were completely fine (around 1.3 MOA from me shooting elbows on the bench).