whichever way you reply, the reply of reply should follow the same flow, its irritating that five replies flow downwards and suddenly 6th one is sitting on the top.
Waisay I like to post my reply at the top so the receiver can quickly read it, just in case the PM is a long one
I use the TOFU style (Text Over, Full Quote Under) which is the default in most email clients... or if the email / PM requires me to answer specific points, I use Inline posting.
Reverse chronology is usually preferred by many end-users these days.
Also, f you use bottom posting (i.e. replying at the end of the original sent message), then you also ought to use the various conventions of trimming and snipping to have the receiver only read relevant parts of the previous messages.