A sufficient number of people bottom post that I installed a plug-in that hides all previous posts unless I expand them out manually. Annoying that I need to do that, but, otherwise, it just takes too long to read the emails. It is then annoying when I want to read something part way through, but, I don't generally complain about it.
My general approach is to reply using whatever method has already been used in posts. So, if there is already a response, I will make my response to be top or bottom based on the previous responses. Consistency within a single message strikes me as a good choice. I make random decisions of top or bottom based on not much when I am the first responder. I am more likely to top post for a person that appears to be a "newbie" since it seems to me that people without exposure to mailing lists are more inclined to to top post. I don't know if that is because that seems to be the way that outlook operates or that is simply what is more natural for them to use. I do know that if I used outlook from work to respond to these messages, it would be a bunch of extra effort to do anything but top post.
-- Andrew Pitonyak My Macro Document: http://www.pitonyak.org/AndrewMacro.odt Info: http://www.pitonyak.org/oo.php