IPNs do need to be enabled in your PayPal account. If they're not, then they generally won't be sent at all.
The URL you set in PayPal doesn't really matter, but one does need to be set there, so we recommend you set it to the "actual" IPN URL. Each request we make to PayPal will set the IPN URL explicitly so multiple sites/URLs can work without issue on one PayPal account.