I'm guilty of it, too: I do refer to Herrmann as "Benny" on the forum at his website, and to Elmer Bernstein by his first name (though it's because I knew Elmer slightly, and addressed him by his first name when we were together), but each case it's an American-born man who was, by all accounts, probably a bit more informal than his European-born colleagues (still, were the Wayback Machine to deposit me on the doorstep of the Warner Bros. Music Department in the late 1930's, I can imagine addressing the decidedly working-class Steiner as "Max," but would find it impossible to call Korngold anything other than "Herr Korngold," or "Maestro."  I'm going to hazard a guess that Alfred Newman would've been comfortable being called by his first name in a social situation.

It wouldn't occur to me to address Rózsa by his given name in any situation (though I'm sure he was always gracious toward those who did), and he did have that honorific of "Doctor" to fall back on.  One should cut Andre Previn a little slack: unlike most, the younger Previn was Rózsa's colleague at MGM, and one whose musicianship Rózsa respected -- for him, surely the greatest of levelers.

In the end, what's most pertinent here is what's pertinent in any sphere: this is a generational argument; as such, anyone who was Rózsa's age or older surely got a pass from the composer, though one doesn't have to be too quick on the uptake to realize that if Rózsa insisted on calling an acquaintance "Mr" or "Miss" or "Mrs," then he probably expected them to reply in kind.