There are alot of different diets out there and what people concider high quality will vary.
Just like dog and cat food. People feeding puppy chow think their dogs getting the best.
Same goes for bunnys, like Purina vs Oxbow.
Salt is a typical mineral alot of animals go to when they feel their body needs it. There will always be the odd animal that will chow down on it no matter what, and if that animal is on a good quality diet then I would take it away, but if not, I would re-concider what I am feeding that animal to make them go nuts for their salt/minerals.
So yes its safe to offer the salt/mineral stones if you keep an eye on it. Sodium and other minerals are key players in daily bodily functions like making the heart beat. So if your bunn feels they need a little lick of their salt, I would offer it.