Think about how stupid it is to ride in single file in the left side of the lane. It just makes no sense. You give up a lot of stopping distance. Do what's safe.
+1
Why would you give up visibility of traffic and road conditions ahead of you just because you don't know the rider in front of you.
If anything, if you don't know or trust the rider's abilities, the last thing you want to do is ride in his tire track in the event that he does something stupid, robbing you of options and maneuvering room.