the tops or colas do ripen earlier than the rest, and in my experience I have taken the top buds WHEN THEY ARE MATURE THO and left the ones on the bottom to mature, I actually liked the lil buds on the bottom after leaving it almost until the first frost. they seem to have a nice flavour to them, I dont know about potency tho....
I would just let the whole thing ripen up if you are unsure, cant do it any harm