There are no rules about expressing - none at all. You can express whenever you want to, starting at birth or waiting weeks or months, or forever.
There is no advantage in waiting, or starting soon, or drawback in either....except that not putting yourself under unnecessary pressure to start expressing (so the maternity nurse/dad/grandma can feed the baby) has to be a good thing, unless (of course) you want this to happen. Your choice.
You also need to know that if you do express (for maternity nurse etc etc), your breasts may feel uncomfortably full because of the missed feed, and this could also effect your supply....esp if you do it routinely, so you regularly leave a long gap between two feeds (this is, after all, how mothers start to wind down their bf, by leaving gaps between feeds). So to protect your supply, and your comfort, you'd do better to express at the time the expressed breastmilk was being given by the maternity nurse. Yes, I know, where is the sense in that, eh?
Anyone being dogmatic about 'having' to express, or the opposite way which would be 'don't express' needs to chill!
Anyone employing a maternity nurse might want to think about getting a cleaner instead - rather less money, rather more use, as you can ask her to do shopping and tidying, and of course cleaning and washing....tasks which a maternity nurse might well think beneath her. I have come across ridiculous situations, where the maternity nurse looks after the baby while the mother goes to the supermarket.
Now that's crazy.