Who wants to scroll through the same message 10 times to read all the replies because you posted at the bottom of the original text? Take an example from Bill Hughes and Jerry Bransford...post your reply at the top. Most newsreaders nest the replies under the original anyway, so it's not like you need a refresher of the topic before reading every reply.
Oh, and while i'm at it...trim the excess off the quoted text. We don't need 3 pages of crap tacked on the end of the old message either. People with grandiose signatures are the main offenders. Every reply adding another page of nothing but signature, and that of course means you have to scroll through 4 pages now just to read a one line reply to a post. Yeesh. /rant