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.
The captain sets the tone of the ship, and considering Kirk was routinely making (or allowing) racial jokes argarding Vulcans on the bridge, I can imagine Kirk was pretty relaxed about forms of address during routine ship's business.