"...one's personal preference does not a grammar rule make."

I'd say it does make a grammar rule, but that acceptance of the rule by other people is by no means guaranteed.

I do suffer from an internal conflict. On the one hand, I believe everyone has his own grammar and "English Grammar" is merely a description of a consensus; "ungrammatical" is therefore only relative, and as Bunn says the "ungrammatical" could be perfectly grammatical in another context or dialect. On the other hand, I get annoyed when people get things wrong :-D