I sort of guess it depends on the type of AAR. I think to write a good history book you need to be well in advance (not necessarily at the end) as that allows better selection of the things to emphasise in earlier posts so it comes together in the end (on the logic that you, as the Historian, know what happened). For a narrative, guess it depends on how much you are following a long term plan and how much you are letting the narrative respond to the game. A common style of CK/EU AAR is to base a post (or a short sequence of posts) on the life of a ruler, so that tends to force you to some sort of advance play but not too much.
For gameplay, I've often used the logic of play a session/write it up but even then I often get ahead of myself if I have a good chance to play (or am in the mood to do so). An issue here is with MP, as the AAR can contain information your opponent could use. I think with the AGEOD games, given the extreme seasonality of some of their games, there is a sort of convention of play the first campaign season before starting the AAR (on the grounds that you may set something in train 4-5 turns before you want your opponent to be aware of what you are up to).
I tend to end my games before I end my AArs, but with a decent set of screenshots (some annotated if needs be), I can usually remember what I was up to - not always, I do sometimes look at a screenshot and struggle to remember just why it seemed useful to take at the time
