Summary: There's a long held myth in duck calling that goes something like this, " Once you've gotten the ducks attention and they are coming towards your spread, stop calling and let 'em work on their own or you'll end up spooking them." Assuming that you are a competent duck caller, there are two reasons why this myth is incorrect.
First, if you've ever had the opportunity to watch and listen to birds in the wild one thing becomes very clear. The ducks that are on the ground or in the water do not suddenly stop calling just because a new group of birds is joining them. In fact, many times they get even louder and noisier as the group in the air communicates with the group that is already there. When you suddenly quit calling it doesn't sound natural. Rather than stop calling to early, adjust your volume and your calling sequences.
Second, calling until you're ready to take the shot reduces your chances of loosing your ducks to another group of duck hunters who may also be calling at your flock. If you hunt in a highly competitive area, like many duck hunters do, your chances of loosing ducks to other competent and aggressive duck callers greatly increases when you stop calling too early. Instead of ending your calling to soon, continue calling until you're ready to take the shot.
But remember, you must really sound like a real duck to implement the above.