Ordinary dangling tags particularly when worn by active dogs, long coated breeds and sports dogs can lead to accidental hair taggles, injury and/or choking, which is why dog’s are not allowed to complete in dog agility whilst wearing dangling tags in the UK! But it is not just sporting dog’s that can get into trouble. I know of one tragic case of a puppy left unsupervised in a dog crate, and the puppy got her bone shaped tag caught in an upper bar, and was found dead by her distraught owner. The puppy had died due to hanging via the tag. For this very reason I cannot recommend regular dangling tags, particularly those which are bone shaped.
Dogs should not be left unsupervised in crates with dangling tags. Particularly bone shaped tags.
There are tags that can be safety left on dogs, please view Dogs ID & Law Article for further information on flat tags which do not dangle.