For those who may not know (I didn't always), don't use "reply" in this case either, because that preserves headers which will cause many mail readers to put the message in the old thread, despite the change of subject line... (I almost forgot for this message.)
But really, it's all the same concept--being polite to the people whom you're asking for help. Take time to be clear with your question, use a relevant concise subject line, quote helpfully not lazily in responses, don't hijack threads.
Top-post annoys me somewhat because of the backward flow when reading those questions. But quoting a whole message with extensive quotes in it already, and then posting some tiny new bit at the end--sheesh--I don't read those. If there's multiple levels of quotes showing, and I have to scroll to get to new content, I just move on instead because right then I assume the poster is just too fucking lazy for me to bother with.