To be honest, I normally forget. It's not intentional, and I usually don't notice because etsy doesn't put it in my face unless I log out for some reason and then log back in. When I see that sentence at the top of the page reminding me to leave feedback, that's when I normally do it. I wish it would show it even when logged in.
So as a buyer I leave feedback as soon as I receive the item, but as a seller I forget to unless I'm forced to log back in and see the reminder.