As far as I know, trimming onion seedlings is done b4 transplanting just to keep the plants from getting all tangled up (and maybe to send some more energy to the roots). I don't think trimming the tops before this is necessary. But thats just my experience (and reading).

Kim; I do not know if it helps, but I had a mushroom onion omelet this morning made with trimmings and it was great. As for trimming I have trimmed onion and leeks for different reasons and really could not tell if it hurt them, I have always had good crops trimmed or not.

Curt :-)

You can trim onions.No harm will be done.
You can actually trim them down to ground like chives
and they will grow back but you would'nt want to do that.

I will let them just grow and give them a final haircul just before
planting. If they are a little thinner than pencil, you can
harden them up and plant. Look around your garden ceenters.
When they start selling onion sets and little pearl onions
you can plant yours in the garden.
Fros or freez is not going to bean issue with onions.

A NATURAL SIGN:
When you seed those wild onions growin everywhere, you can plant your onion sets too.