I have never understood why some say trim them, it seems to go the principles of plant culture, after all the roots and leaves, particularly those of seedlings are vital to their development.
The roots to seek out and supply the nutrients and the leaves to photosynthesise them.
I think the trouble that the top growers go to drape the leaves over supports so that each leave gets maximum light and the special brews they use to feed is testament to that.
But then again I was always told if you find a way that works, don't change it until you find a better method.
I never cut the leaves or trim the roots, but then again I grow my leeks on in individual pots until planting out time.