I always address my parents in plural. And i always thought this to be the case. Until i noticed my father addressing his mother in singular. When asked, he didn't give me a very convincing answer. Some of my friends also do the same.
I'm not comfortable with this, and would never even dream of this. Guess i'm so used to this. I understand this sounds weird, but one reason why i preferred to converse in English than Kannada was this reason.