I know my grammar is not perfect, but bad grammar, diminishes my opinion of someone. With the caveat that I find regional linguistic traits and dialects to be wonderful.
I recently joined an online dating site. If a fellow has more than two typos or poor uses of grammar in his profile, I stop reading. If I get a message asking to meet with spelling and grammatical mistakes I politely decline.
I know I am guilty of saying "I was thinking in my head." I am trying to break that habit.