I've noticed that when going through posts, I'll read something and think, 'oh this person is from x, just by the words and phrases they use. I'll sometimes be able to localise this to a more specific region. I never look at usernames when I'm reading unless something really stands out to me, so I generally don't have any prior knowledge of the poster influencing me. Of course I can't be 100% sure but I would put a bet on being right, most of the time.
Do you ever recognise a particular region through how a poster writes? Are you conscious of how you write, making sure not to use colloquialisms unique to your area or do you not think about it, or do it anyway?