Old professor speaking here: I look upon these threads as group discussions. One of the first rules of good manners in such a situation is that you LISTEN(in this case read) what the person(s) have to say before responding. If you have nothing to ad, then just "second" what is said, or say nothing at all. So yes, read the entire thread before replying. (Just use good sense, however, some threads take an entire evening to read through)
Just my tuppence worth.