Yes, I write it on the bill,
and
Yes, we have had to use it. Credit card put the payment in the wrong account, if not we would have been out of luck, house payment not applied to right account that one we had to go round and round with them, even with the confirmation number...and they still would not admit that they were wrong.
I have a bit of OCD, I print out all the reservations, confirmation numbers when we travel, one set in DH's backpack and one set in my handbag.