Yes.  Preacher curls are safe.  Some will raise a concern due to the stress that it can put on the elbow. Now, there is some truth to this but it doesn't mean it's dangerous, just that you need to follow precautions.  The main point is to use light weight.  This is an isolation movement.  Don't try to use excessively heavy loads.  80%1RM (8 rep max) should be the max weight you should use.