I think we can see it's different by practice. It depends on their equipment. 4 years ago when I was pregnant with my son, I had to go with a full bladder.
This time, they told me NOT to come with a full bladder because they have new machines that are so sentitive, a full bladder will actually hinder the scan.
If you need a full bladder, they will tell you. But like a pp said, by 20 weeks, it prob won't be necessary.