Is it safe to use a pre-emergence herbacide in my onion bed prior to planting sets?
safe for what? Safe for the onions? Yes. Safe for your consumption? You would have to read the label and if it says so, then ask yourself if you trust that info.
Some folks use corn gluten, sometimes available at feed stores as an organic pre-emergent. I would trust that to be safe to eat since it is just part of the corn kernel, but pretty much anything else I would avoid in food crop areas. That is just me though.
I grow many beds of alliums, and do not need any herbicide. I mulch with thick, loose organic matter and leave that on thru winter and summer. Any weeds that do come thru can be plucked out easily. The sides of my berm beds are covered with paper and several inches of mulch, too.
I agree with username's perspective, and used corn gluten on beds that had more soil/sun exposure. It worked pretty well for the first try. A good thing to use where such as Sunberry or Yarrow have gone to seed before i got a grip on the beds!!
Anytime i can avoid a "cide" in the gardens that i feed the family from, the better. Just another opinion.