AFAIK you can join the ACO any time you want. I left my card at home one year and they were able to issue me a temporary one, so your membership is valid and usable immediately you have coughed up the dosh.
Every time I go with good intentions of having a hot shower at some point during the week, but there's always something more important to do. So I regard the major benefit as the savings on the tickets. Mind you, I could have ate my dinner off the ACO crappers at test weekend. Paper too!
