Onions do not need the tops cut off at any point. Iif they get a flower stem coming up on them, you want to remove that as it drains energy off of the bulb that is developing. When the onions mature you can bend the tops down to start the drying process. We always pull them when they stop growing and the stems start browning, shake off as much dirt as you can and braid them into ropes to hang from the porch. This way they air dry and are not resting on any surface that would hold moisture and cause rot.