It's completely up to you -- we make no rules about it and we work with whatever situation you present us with. However, we would not let it go until the last minute, because rushing things can cause errors, and also because our workload might not give us time to get everything done in time for you to board the airplane! And in certain somewhat rarely-occurring circumstances, the date you signed up can have an effect on your boarding priority -- so, all other things being equal, it's better to sign up sooner rather than later!