A couple of years ago we got an email from the union reminding us to check credentials on all JSers even if they have a seat in the back. The story goes that a crew was getting a line check from a fed. JSer checks in with the Captain, Captain tells him to grab a seat. After the formalities are done fed goes back to get JSers info since he is an XCM. Guy didnt have his license or medical on him so the Captain gets violated.

After that email I always check.