Wasn't Gene Roddenberry also trying to show a more informal version of the Navy in regards to Starfleet tradition? Hence the lack of saluting going on. Aside from Kirk, there was very little in the ways of using rank as title. It was generally "Mr. Sulu, Mr. Chekov, Mr. Scott" and so on.
That seemed to be the norm, with the exception of Uhura. I think he always called her Lieutenant. He did refer to more senior officers, than him, by their rank with a few exceptions. Looking back, there just seemed to be no general rule for things like that.