Higgins has been preaching this as well, and I still can't get on board. He said up to Labor Day is good timing. The claim is that the younger smaller plants taste better. I'm sure they do. But my July brassicas get huge and still get totally hammered. I'm a throw and grow plotter, so I have to time the rains to get a good germ. First chances of good rain after the 1st week of July and I'm starting to spread seed. Occasionally I'm still trying to get a good stand in the first couple weeks of August, and I just can't get the growth out of those. I'm sticking with July.
Edit, to be accurate, he said for every 100 miles north of where he lives in central IL, to move a week earlier than Labor Day. I'm still sticking to July, and I don't think it matters all that much which type of the more popular brassicas you go with, as far as maturities. Maybe some really early maturing types need to be waited on. I plant 4 types and haven't seen 1 bolt yet. Needless to say, weather will have the final say. And you can pour the N to late planted plots to hurry them along, which I don't. So I guess it all depends.