I correct people on proper grammar, or at least I wanted to the other day!
I dunno, I think it depends on the situation. Where I come from, if you corrected someone's grammer they would probably think you were stuck-up and would either (a) ignore you, (b) insult you, or (c) beat you up. To be honest, I don't blame them... if someone wants to use incorrect grammer, that's their own business.