The similar tail numbers are okay when you're at an airport which is familiar with them. At riddle in prescott we would say "riddle 12" and from what I understand that's what pan am does as well. We even used the "riddle #" call sign at local airports like deer valley. The problem is that when you go to an airport which is not familiar with you and you use the regular language of "one two echo romeo", it is easy to confuse with other "echo romeo" aircraft since it is at the end of the identification. When I used to take a riddle plane somewhere and heard another riddle plane on the same frequency I would always use the full tail number and, in the case of an uncontrolled field, I would advise other aircraft on frequency that there were in fact multiple "echo romeo" aircraft in the area. The only advantage I ever found (if you want to call it an advantage) is that many controllers and weather briefers would recognize you as a riddle student.