To sum up lots of mentions in various threads, it seems that those of us who start early and must keep onion seedlings under lights for months favor trimming. Those who have lots of natural light to work with, either from a nice climate or a greenhouse, often favor not trimming.

I am among the trimmers, mostly to keep the January-sown plants manageable and upright, and to increase light penetration to new leaves. In the next couple of weeks, when I can put the onions outside at least a little bit, they will stiffen up nicely and I'll stop trimming.

I too trim. It's still a bit too early for them to go into the ground here in Vermont plus they are absolutely delicious in salads and other foods. I've not run side-by-side studies to determine the size of the bulbs but they sure get leggy if I don't trim them.