Depends on the thread type. Normally, I usually post in Other Voting Polls, so I don't really need to think my posts through. But when I post in a discussion-oriented thread, then I think my post over, as to not give my opponents something to trip me up with. I like to make some sort of impression with my posts, so that people can remember them, rather than to look at them and forget.
Also, I read through my posts so that I make sure I don't go too far off-topic, especially with long posts, like this one.