Always taught on larger multi-crew aircraft that you use crew positions in the aircraft - good advice when 4 of you have the same Christian name on a crew as once happened to me (I was one of them). Also, do not be called Mark - especially on an aircraft with smoke floats!
Bottom line - during quiet periods, or when saying thank-you then Christian names can be OK. However, when the workload goes up then crew position should be the order of the day.
I only use Sir when the Staish is flying....
For thread drift... are there any other names that have/can cause confusion on a crew apart from Mark?