Let's say that all else being equal, procrastination is a bad writing technique. Even if some people "get away with it," they'd have written better papers if they'd gotten started earlier and had more time for research, reflection, and revision.
I don't have evidence for this, but it's the kind of commonsense notion that I expect is hard to let go of without direct evidence against it. Or even then.
It's still not all that useful to tell students not to procrastinate. I mean, it doesn't hurt to give them an idea of how much time you expect them to spend on the assignment. If they've never had to write something they couldn't do in one sitting, they could benefit from a description of a different writing process that might work better. But if the main advice is "start this paper as soon as you get the assignment sheet," then what will the student do?
Those who actually take the advice will most likely use the same writing process they would have used at the last minute. They'll merely move up the timeframe. So instead of writing it all in one sitting the night before, they'll write it all in one sitting the night they are given the assignment. Which is worse, by the way, since those who wait longer will have probably learned more about the subject in the meantime.