Oh, I so wish I could sow the seedlings right into the ground and not give them haircuts, but alas, I live in the most brutal of growing zones, so I have to do things differently. When you start seedlings indoors they are prone to damping off and dying, cutting back the tops of onions helps them put energy into strong root development and doesn't hurt them at all, I've grown them this way lots of times, as long as you don't cut them back too far or aggressively. I don't do it to any other seedlings but the onions.

!PIZZA
!BBH