I voted for reading the whole thread. If I want to respond to an early message and haven't finished reading the thread yet, I usually earmark stuff as I go along and use the quote-plus button, which snags the messages I'm reacting to and assembles them into the editing box when I hit the "post reply" button.

I also have a habit of using the "Preview Post" button to not only see how my composition looks, but to see if someone beat me to the punch.

Regards,
Rex.