When I was young and foolish, I corrected the spelling and grammar of a letter my best friend had sent. It was not well received.
I do pay attention to the structure of casual speech, though not for the purpose of criticism. I just find it interesting how familiarity and a comfortable atmosphere influence the elements of communication. For instance, I am trying to nail down the meaning and function of the term "harn" which crops up frequently among a small group of forum friends with whom I have become acqainted.