When we delete an account online, we usually receive a confirmation email a few days or weeks after we start the deletion process. This means that if we delete the alias used, it will not be possible to receive the confirmation email; if we don't delete it, we may forget to clean it up later.
So it might be a good idea to delete the identity and alias with a delay (7 days, 30 days or 2 months, for example).
This way, nothing is forgotten, the safe is tidy and we have peace of mind.