The IPN handler URL is automatically set by the Paypal IPN module. So, it should never be incorrect unless Paypal does a Zig or Zag with API functionality (which they have been rumored to do...).
That said, here are good general rules of thumb with Paypal IPN:
NEVER change the order status manually in the cart before the payment is KNOWN ACCEPTED and cleared in Paypal. Ever.
Why?? Because it opens up a vulnerability for filling an order, then not receiving payment.
After that - if you ship via Paypal's tools, let Paypal manage the order status changes.
This will let Paypal continue to update the order status. So, if you get an order cancellation the order status will change.
Paypal was one of , if not the first to provide a rather complete set of order status notifications across the web. Not unsurprising given their linkage with eBay. As long as you have not manually changed the order status, they will keep the cart advised of changes for you. Once you have changed it however, things get a bit hairy and you are safer not to count on any more updates.
Of course if you don't use them for shipping - just remember to update the order status in the cart admin as usual and move on..
David