For a long time my 1/2 TB gelding needed a chain under his chin. He just always had to be going somewhere, very forward thinking. He was light as a feather with the chain even tho neither one of us had pulled on it. He just seemed to know.
We're pretty much required to use them with all the horses at my school. If you know the horse and know they don't need one (Juanita's just one of the horses that I know doesn't), you're welcome to go without..But, if they get loose or run you over, you're going to get blamed for not using a chain. They're all worn under the nose aside from in the farrier shop, because under the nose basically means up if the horse is being a complete ass and gets shanked, so it goes over the nose in the shop to help prevent the horses from rearing while someone is under them with a foot.
I always put one on any of the horses I bring in for farrier or horse health (aka, simple vet class). I rarely have to use it on most, but some of them are runaways (they always want the grass since they don't get any grazing time), studs (even though some are nice and calm as a gelding), and hormonal mares that want to swing their butt around and kick any other horse. While I'd expect these horses to all be deadbroke beginner horses...they're not, and every single one of them has atleast one issue that deserves either a chain or a whip.
I'll put one on a horse I don't know (out in the industry, training/hop on and fix a small problem, etc) for the just in case factor..I'd rather have it and not need it than need it and not have it, ending up in me or someone else getting hurt.