I always do! It's delicious. I assume you are using ruminent bones, thus the fat is high in SAT FAT.
Also, I'm assuming you are making your broth at a relatively low temperature, so the fat should be fine.
Why would it not be safe?
I allow my broth to cool, remove the layer of fat, rinse in cold water quickly and pat it try with paper towel. Then I put it in a glass jar and keep it in the fridge, using the tallow to cook cauliflower, mushrooms, etc in a pan.
I'm a bit picky, so it depends. If I roasted and rinsed the bones beforehand and skimmed throughout, then I use the fat and it is excellent. However, if I just throw some bones in a pot and ignore it for the whole time, it gets kind of a scummy, gummy flavour, especially with beef bones. It's worth it to treat it with a little love, because it's a great fat source!